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    <link:label id="lab_art_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8AA12E55B5795B44B98F46499B06AD57_terseLabel_en-US" xlink:label="lab_art_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8AA12E55B5795B44B98F46499B06AD57" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Additional accordion option</link:label>
    <link:label id="lab_art_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8AA12E55B5795B44B98F46499B06AD57_label_en-US" xlink:label="lab_art_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8AA12E55B5795B44B98F46499B06AD57" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Line Of Credit Facility, Accordion Feature, Increase Limit</link:label>
    <link:label id="lab_art_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8AA12E55B5795B44B98F46499B06AD57_documentation_en-US" xlink:label="lab_art_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8AA12E55B5795B44B98F46499B06AD57" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Line of Credit Facility, Accordion Feature, Increase Limit</link:label>
    <link:loc xlink:href="art-20180331.xsd#art_LineOfCreditFacilityAccordionFeatureIncreaseLimit" xlink:label="loc_art_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8AA12E55B5795B44B98F46499B06AD57" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_art_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8AA12E55B5795B44B98F46499B06AD57" xlink:to="lab_art_LineOfCreditFacilityAccordionFeatureIncreaseLimit_8AA12E55B5795B44B98F46499B06AD57" xlink:type="arc" />
    <link:label id="lab_art_DebtInstrumentExtensionTerm_623173A97451796DF55F46499B06305C_terseLabel_en-US" xlink:label="lab_art_DebtInstrumentExtensionTerm_623173A97451796DF55F46499B06305C" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Extension term</link:label>
    <link:label id="lab_art_DebtInstrumentExtensionTerm_623173A97451796DF55F46499B06305C_label_en-US" xlink:label="lab_art_DebtInstrumentExtensionTerm_623173A97451796DF55F46499B06305C" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Extension Term</link:label>
    <link:label id="lab_art_DebtInstrumentExtensionTerm_623173A97451796DF55F46499B06305C_documentation_en-US" xlink:label="lab_art_DebtInstrumentExtensionTerm_623173A97451796DF55F46499B06305C" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Debt Instrument, Extension Term</link:label>
    <link:loc xlink:href="art-20180331.xsd#art_DebtInstrumentExtensionTerm" xlink:label="loc_art_DebtInstrumentExtensionTerm_623173A97451796DF55F46499B06305C" xlink:type="locator" />
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    <link:label id="lab_us-gaap_ProceedsFromLinesOfCredit_B4C2E47892D2FD7EB24446499B07558C_terseLabel_en-US" xlink:label="lab_us-gaap_ProceedsFromLinesOfCredit_B4C2E47892D2FD7EB24446499B07558C" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Proceeds from lines of credit</link:label>
    <link:label id="lab_us-gaap_ProceedsFromLinesOfCredit_B4C2E47892D2FD7EB24446499B07558C_label_en-US" xlink:label="lab_us-gaap_ProceedsFromLinesOfCredit_B4C2E47892D2FD7EB24446499B07558C" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Proceeds from Lines of Credit</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProceedsFromLinesOfCredit" xlink:label="loc_us-gaap_ProceedsFromLinesOfCredit_B4C2E47892D2FD7EB24446499B07558C" xlink:type="locator" />
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    <link:label id="lab_us-gaap_RepaymentsOfLinesOfCredit_9C5DBC5447B2B737D6DD46499B07D853_terseLabel_en-US" xlink:label="lab_us-gaap_RepaymentsOfLinesOfCredit_9C5DBC5447B2B737D6DD46499B07D853" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Repayments of lines of credit</link:label>
    <link:label id="lab_us-gaap_RepaymentsOfLinesOfCredit_9C5DBC5447B2B737D6DD46499B07D853_label_en-US" xlink:label="lab_us-gaap_RepaymentsOfLinesOfCredit_9C5DBC5447B2B737D6DD46499B07D853" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Repayments of Lines of Credit</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepaymentsOfLinesOfCredit" xlink:label="loc_us-gaap_RepaymentsOfLinesOfCredit_9C5DBC5447B2B737D6DD46499B07D853" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_RepaymentsOfLinesOfCredit_9C5DBC5447B2B737D6DD46499B07D853" xlink:to="lab_us-gaap_RepaymentsOfLinesOfCredit_9C5DBC5447B2B737D6DD46499B07D853" xlink:type="arc" />
    <link:label id="lab_us-gaap_RepaymentsOfDebt_7A053F3843AE464E708A46499B078E03_terseLabel_en-US" xlink:label="lab_us-gaap_RepaymentsOfDebt_7A053F3843AE464E708A46499B078E03" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Indebtedness repaid</link:label>
    <link:label id="lab_us-gaap_RepaymentsOfDebt_7A053F3843AE464E708A46499B078E03_label_en-US" xlink:label="lab_us-gaap_RepaymentsOfDebt_7A053F3843AE464E708A46499B078E03" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Repayments of Debt</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RepaymentsOfDebt" xlink:label="loc_us-gaap_RepaymentsOfDebt_7A053F3843AE464E708A46499B078E03" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_RepaymentsOfDebt_7A053F3843AE464E708A46499B078E03" xlink:to="lab_us-gaap_RepaymentsOfDebt_7A053F3843AE464E708A46499B078E03" xlink:type="arc" />
    <link:label id="lab_art_LineOfCreditFacilityIncreaseDecreaseMaximumBorrowingCapacity_C88C7F8E5F41213B8F6146499B071880_terseLabel_en-US" xlink:label="lab_art_LineOfCreditFacilityIncreaseDecreaseMaximumBorrowingCapacity_C88C7F8E5F41213B8F6146499B071880" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Increase in credit commitments</link:label>
    <link:label id="lab_art_LineOfCreditFacilityIncreaseDecreaseMaximumBorrowingCapacity_C88C7F8E5F41213B8F6146499B071880_label_en-US" xlink:label="lab_art_LineOfCreditFacilityIncreaseDecreaseMaximumBorrowingCapacity_C88C7F8E5F41213B8F6146499B071880" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Line Of Credit Facility, Increase (Decrease) Maximum Borrowing Capacity</link:label>
    <link:label id="lab_art_LineOfCreditFacilityIncreaseDecreaseMaximumBorrowingCapacity_C88C7F8E5F41213B8F6146499B071880_documentation_en-US" xlink:label="lab_art_LineOfCreditFacilityIncreaseDecreaseMaximumBorrowingCapacity_C88C7F8E5F41213B8F6146499B071880" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Line Of Credit Facility, Increase (Decrease) Maximum Borrowing Capacity</link:label>
    <link:loc xlink:href="art-20180331.xsd#art_LineOfCreditFacilityIncreaseDecreaseMaximumBorrowingCapacity" xlink:label="loc_art_LineOfCreditFacilityIncreaseDecreaseMaximumBorrowingCapacity_C88C7F8E5F41213B8F6146499B071880" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_art_LineOfCreditFacilityIncreaseDecreaseMaximumBorrowingCapacity_C88C7F8E5F41213B8F6146499B071880" xlink:to="lab_art_LineOfCreditFacilityIncreaseDecreaseMaximumBorrowingCapacity_C88C7F8E5F41213B8F6146499B071880" xlink:type="arc" />
    <link:label id="lab_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_279AEB168857CEC4FA8A46499B07B86C_verboseLabel_en-US" xlink:label="lab_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_279AEB168857CEC4FA8A46499B07B86C" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:type="resource" xml:lang="en-US">Basis spread on variable rate</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_279AEB168857CEC4FA8A46499B07B86C" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_279AEB168857CEC4FA8A46499B07B86C" xlink:to="lab_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_279AEB168857CEC4FA8A46499B07B86C" xlink:type="arc" />
    <link:label id="lab_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_DBCF8BB7A3060E0F7CA146499B074AF7_terseLabel_en-US" xlink:label="lab_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_DBCF8BB7A3060E0F7CA146499B074AF7" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Commitment fee percentage</link:label>
    <link:label id="lab_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_DBCF8BB7A3060E0F7CA146499B074AF7_label_en-US" xlink:label="lab_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_DBCF8BB7A3060E0F7CA146499B074AF7" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Line of Credit Facility, Commitment Fee Percentage</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LineOfCreditFacilityCommitmentFeePercentage" xlink:label="loc_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_DBCF8BB7A3060E0F7CA146499B074AF7" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_DBCF8BB7A3060E0F7CA146499B074AF7" xlink:to="lab_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_DBCF8BB7A3060E0F7CA146499B074AF7" xlink:type="arc" />
    <link:label id="lab_art_LineOfCreditFacilityThresholdForCommitmentFee_B200764ECE24AED10D0F46499B0700BB_terseLabel_en-US" xlink:label="lab_art_LineOfCreditFacilityThresholdForCommitmentFee_B200764ECE24AED10D0F46499B0700BB" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Commitment threshold</link:label>
    <link:label id="lab_art_LineOfCreditFacilityThresholdForCommitmentFee_B200764ECE24AED10D0F46499B0700BB_label_en-US" xlink:label="lab_art_LineOfCreditFacilityThresholdForCommitmentFee_B200764ECE24AED10D0F46499B0700BB" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Line Of Credit Facility, Threshold For Commitment Fee</link:label>
    <link:label id="lab_art_LineOfCreditFacilityThresholdForCommitmentFee_B200764ECE24AED10D0F46499B0700BB_documentation_en-US" xlink:label="lab_art_LineOfCreditFacilityThresholdForCommitmentFee_B200764ECE24AED10D0F46499B0700BB" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Line Of Credit Facility, Threshold For Commitment Fee</link:label>
    <link:loc xlink:href="art-20180331.xsd#art_LineOfCreditFacilityThresholdForCommitmentFee" xlink:label="loc_art_LineOfCreditFacilityThresholdForCommitmentFee_B200764ECE24AED10D0F46499B0700BB" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_art_LineOfCreditFacilityThresholdForCommitmentFee_B200764ECE24AED10D0F46499B0700BB" xlink:to="lab_art_LineOfCreditFacilityThresholdForCommitmentFee_B200764ECE24AED10D0F46499B0700BB" xlink:type="arc" />
    <link:label id="lab_art_DebtInstrumentAssetsUsedinCalculationofCollateralAmount_F1E0DD5583942AA9470D46499B07980F_terseLabel_en-US" xlink:label="lab_art_DebtInstrumentAssetsUsedinCalculationofCollateralAmount_F1E0DD5583942AA9470D46499B07980F" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Gross value of assets included in calculations under credit agreement</link:label>
    <link:label id="lab_art_DebtInstrumentAssetsUsedinCalculationofCollateralAmount_F1E0DD5583942AA9470D46499B07980F_label_en-US" xlink:label="lab_art_DebtInstrumentAssetsUsedinCalculationofCollateralAmount_F1E0DD5583942AA9470D46499B07980F" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Assets Used in Calculation of Collateral Amount</link:label>
    <link:label id="lab_art_DebtInstrumentAssetsUsedinCalculationofCollateralAmount_F1E0DD5583942AA9470D46499B07980F_documentation_en-US" xlink:label="lab_art_DebtInstrumentAssetsUsedinCalculationofCollateralAmount_F1E0DD5583942AA9470D46499B07980F" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Debt Instrument, Assets Used in Calculation of Collateral Amount</link:label>
    <link:loc xlink:href="art-20180331.xsd#art_DebtInstrumentAssetsUsedinCalculationofCollateralAmount" xlink:label="loc_art_DebtInstrumentAssetsUsedinCalculationofCollateralAmount_F1E0DD5583942AA9470D46499B07980F" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_art_DebtInstrumentAssetsUsedinCalculationofCollateralAmount_F1E0DD5583942AA9470D46499B07980F" xlink:to="lab_art_DebtInstrumentAssetsUsedinCalculationofCollateralAmount_F1E0DD5583942AA9470D46499B07980F" xlink:type="arc" />
    <link:label id="lab_us-gaap_DebtInstrumentCollateralAmount_3ADADB6F98DBC51A6AEB46499B078D19_terseLabel_en-US" xlink:label="lab_us-gaap_DebtInstrumentCollateralAmount_3ADADB6F98DBC51A6AEB46499B078D19" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Borrowing base collateral value</link:label>
    <link:label id="lab_us-gaap_DebtInstrumentCollateralAmount_3ADADB6F98DBC51A6AEB46499B078D19_label_en-US" xlink:label="lab_us-gaap_DebtInstrumentCollateralAmount_3ADADB6F98DBC51A6AEB46499B078D19" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Collateral Amount</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentCollateralAmount" xlink:label="loc_us-gaap_DebtInstrumentCollateralAmount_3ADADB6F98DBC51A6AEB46499B078D19" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_DebtInstrumentCollateralAmount_3ADADB6F98DBC51A6AEB46499B078D19" xlink:to="lab_us-gaap_DebtInstrumentCollateralAmount_3ADADB6F98DBC51A6AEB46499B078D19" xlink:type="arc" />
    <link:label id="lab_art_DebtInstrumentCovenantLeverageRatioMaximum_3984D5E1CC9C0CFE4F6E46499B072C96_terseLabel_en-US" xlink:label="lab_art_DebtInstrumentCovenantLeverageRatioMaximum_3984D5E1CC9C0CFE4F6E46499B072C96" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Maximum leverage ratio</link:label>
    <link:label id="lab_art_DebtInstrumentCovenantLeverageRatioMaximum_3984D5E1CC9C0CFE4F6E46499B072C96_label_en-US" xlink:label="lab_art_DebtInstrumentCovenantLeverageRatioMaximum_3984D5E1CC9C0CFE4F6E46499B072C96" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Leverage Ratio, Maximum</link:label>
    <link:label id="lab_art_DebtInstrumentCovenantLeverageRatioMaximum_3984D5E1CC9C0CFE4F6E46499B072C96_documentation_en-US" xlink:label="lab_art_DebtInstrumentCovenantLeverageRatioMaximum_3984D5E1CC9C0CFE4F6E46499B072C96" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Leverage Ratio, Maximum</link:label>
    <link:loc xlink:href="art-20180331.xsd#art_DebtInstrumentCovenantLeverageRatioMaximum" xlink:label="loc_art_DebtInstrumentCovenantLeverageRatioMaximum_3984D5E1CC9C0CFE4F6E46499B072C96" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_art_DebtInstrumentCovenantLeverageRatioMaximum_3984D5E1CC9C0CFE4F6E46499B072C96" xlink:to="lab_art_DebtInstrumentCovenantLeverageRatioMaximum_3984D5E1CC9C0CFE4F6E46499B072C96" xlink:type="arc" />
    <link:label id="lab_art_DebtInstrumentCovenantBaseCoverageRatioMinimum_072223C85895FE70654846499B070B46_terseLabel_en-US" xlink:label="lab_art_DebtInstrumentCovenantBaseCoverageRatioMinimum_072223C85895FE70654846499B070B46" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Minimum base coverage ratio</link:label>
    <link:label id="lab_art_DebtInstrumentCovenantBaseCoverageRatioMinimum_072223C85895FE70654846499B070B46_label_en-US" xlink:label="lab_art_DebtInstrumentCovenantBaseCoverageRatioMinimum_072223C85895FE70654846499B070B46" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Base Coverage Ratio, Minimum</link:label>
    <link:label id="lab_art_DebtInstrumentCovenantBaseCoverageRatioMinimum_072223C85895FE70654846499B070B46_documentation_en-US" xlink:label="lab_art_DebtInstrumentCovenantBaseCoverageRatioMinimum_072223C85895FE70654846499B070B46" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:type="resource" xml:lang="en-US">Debt Instrument, Covenant, Base Coverage Ratio, Minimum</link:label>
    <link:loc xlink:href="art-20180331.xsd#art_DebtInstrumentCovenantBaseCoverageRatioMinimum" xlink:label="loc_art_DebtInstrumentCovenantBaseCoverageRatioMinimum_072223C85895FE70654846499B070B46" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_art_DebtInstrumentCovenantBaseCoverageRatioMinimum_072223C85895FE70654846499B070B46" xlink:to="lab_art_DebtInstrumentCovenantBaseCoverageRatioMinimum_072223C85895FE70654846499B070B46" xlink:type="arc" />
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    <link:label id="lab_us-gaap_OperatingIncomeLoss_9AAB043B241E886FC14D4B9AC2F21E48_label_en-US" xlink:label="lab_us-gaap_OperatingIncomeLoss_9AAB043B241E886FC14D4B9AC2F21E48" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Operating Income (Loss)</link:label>
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    <link:label id="lab_art_TwoThousandFifteenSeniorSecuredCreditFacilitiesMemberMember_C87A4F872F992A8FF71A46499A4BEFBF_terseLabel_en-US" xlink:label="lab_art_TwoThousandFifteenSeniorSecuredCreditFacilitiesMemberMember_C87A4F872F992A8FF71A46499A4BEFBF" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">2015 Revolving Credit Facility</link:label>
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    <link:label id="lab_art_GainLossonExtinguishmentofDebtNoncash_8FCF201BE5239ACD5ABB4B1335720829_negatedTerseLabel_en-US" xlink:label="lab_art_GainLossonExtinguishmentofDebtNoncash_8FCF201BE5239ACD5ABB4B1335720829" xlink:role="http://www.xbrl.org/2009/role/negatedTerseLabel" xlink:type="resource" xml:lang="en-US">Loss on debt extinguishment and modification, non-cash</link:label>
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    <link:label id="lab_us-gaap_ProvisionForDoubtfulAccounts_0833CD389918ED7F2B974B133572359E_label_en-US" xlink:label="lab_us-gaap_ProvisionForDoubtfulAccounts_0833CD389918ED7F2B974B133572359E" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Provision for Doubtful Accounts</link:label>
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    <link:label id="lab_art_SaleLeasebackTransactionRepaymentFinancingActivities_917B4E8F5D8F3B8DF4664B133572C905_negatedTerseLabel_en-US" xlink:label="lab_art_SaleLeasebackTransactionRepaymentFinancingActivities_917B4E8F5D8F3B8DF4664B133572C905" xlink:role="http://www.xbrl.org/2009/role/negatedTerseLabel" xlink:type="resource" xml:lang="en-US">Repayment of sale-leaseback financing obligations</link:label>
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    <link:label id="lab_us-gaap_NetCashProvidedByUsedInFinancingActivities_F5BCF521AD66E64147184B1335723F5F_totalLabel_en-US" xlink:label="lab_us-gaap_NetCashProvidedByUsedInFinancingActivities_F5BCF521AD66E64147184B1335723F5F" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:type="resource" xml:lang="en-US">Net cash provided by (used in) financing activities</link:label>
    <link:label id="lab_us-gaap_NetCashProvidedByUsedInFinancingActivities_F5BCF521AD66E64147184B1335723F5F_label_en-US" xlink:label="lab_us-gaap_NetCashProvidedByUsedInFinancingActivities_F5BCF521AD66E64147184B1335723F5F" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Net Cash Provided by (Used in) Financing Activities</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_F5BCF521AD66E64147184B1335723F5F" xlink:type="locator" />
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    <link:label id="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect_3CAB8E03059F038DB0C24B1335722441_terseLabel_en-US" xlink:label="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect_3CAB8E03059F038DB0C24B1335722441" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Net increase (decrease) in cash, cash equivalents and restricted cash</link:label>
    <link:label id="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect_3CAB8E03059F038DB0C24B1335722441_label_en-US" xlink:label="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect_3CAB8E03059F038DB0C24B1335722441" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect_3CAB8E03059F038DB0C24B1335722441" xlink:type="locator" />
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    <link:label id="lab_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_F1B2900B47FF474225464B133572E3CA_terseLabel_en-US" xlink:label="lab_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_F1B2900B47FF474225464B133572E3CA" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Effect of foreign currency translation on cash, cash equivalents and restricted cash</link:label>
    <link:label id="lab_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_F1B2900B47FF474225464B133572E3CA_label_en-US" xlink:label="lab_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_F1B2900B47FF474225464B133572E3CA" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_F1B2900B47FF474225464B133572E3CA" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_F1B2900B47FF474225464B133572E3CA" xlink:to="lab_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_F1B2900B47FF474225464B133572E3CA" xlink:type="arc" />
    <link:label id="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffectAbstract_EFAE03C1BA2987A48F5E4B1335723A7A_terseLabel_en-US" xlink:label="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffectAbstract_EFAE03C1BA2987A48F5E4B1335723A7A" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Cash, cash equivalents and restricted cash:</link:label>
    <link:label id="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffectAbstract_EFAE03C1BA2987A48F5E4B1335723A7A_label_en-US" xlink:label="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffectAbstract_EFAE03C1BA2987A48F5E4B1335723A7A" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect [Abstract]</link:label>
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    <link:label id="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_FAEDEF06CFBB5C92391B4B13357295F8_periodStartLabel_en-US" xlink:label="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_FAEDEF06CFBB5C92391B4B13357295F8" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:type="resource" xml:lang="en-US">Beginning of period</link:label>
    <link:label id="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_FAEDEF06CFBB5C92391B4B13357295F8_label_en-US" xlink:label="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_FAEDEF06CFBB5C92391B4B13357295F8" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_FAEDEF06CFBB5C92391B4B13357295F8" xlink:type="locator" />
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    <link:label id="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_67D2EADFD1D1D70932344B133572CBD7_periodEndLabel_en-US" xlink:label="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_67D2EADFD1D1D70932344B133572CBD7" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:type="resource" xml:lang="en-US">End of period</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_67D2EADFD1D1D70932344B133572CBD7" xlink:type="locator" />
    <link:labelArc order="1" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_67D2EADFD1D1D70932344B133572CBD7" xlink:to="lab_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_67D2EADFD1D1D70932344B133572CBD7" xlink:type="arc" />
    <link:label id="lab_us-gaap_SupplementalCashFlowInformationAbstract_4E897B76A56D4AB6AE6B4B1335725BB8_terseLabel_en-US" xlink:label="lab_us-gaap_SupplementalCashFlowInformationAbstract_4E897B76A56D4AB6AE6B4B1335725BB8" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Supplemental disclosures of cash flows information:</link:label>
    <link:label id="lab_us-gaap_SupplementalCashFlowInformationAbstract_4E897B76A56D4AB6AE6B4B1335725BB8_label_en-US" xlink:label="lab_us-gaap_SupplementalCashFlowInformationAbstract_4E897B76A56D4AB6AE6B4B1335725BB8" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Supplemental Cash Flow Information [Abstract]</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SupplementalCashFlowInformationAbstract" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_4E897B76A56D4AB6AE6B4B1335725BB8" xlink:type="locator" />
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    <link:label id="lab_art_NoncashorPartNoncashAcquisitionFixedAssetsUnderCapitalLeaseObligationsAcquired_9F3B892E3D9D91EFCC224B1335727142_terseLabel_en-US" xlink:label="lab_art_NoncashorPartNoncashAcquisitionFixedAssetsUnderCapitalLeaseObligationsAcquired_9F3B892E3D9D91EFCC224B1335727142" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Acquisition of fixed assets under capitalized lease obligations</link:label>
    <link:label id="lab_art_NoncashorPartNoncashAcquisitionFixedAssetsUnderCapitalLeaseObligationsAcquired_9F3B892E3D9D91EFCC224B1335727142_label_en-US" xlink:label="lab_art_NoncashorPartNoncashAcquisitionFixedAssetsUnderCapitalLeaseObligationsAcquired_9F3B892E3D9D91EFCC224B1335727142" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Noncash or Part Noncash Acquisition, Fixed Assets Under Capital Lease Obligations Acquired</link:label>
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    <link:loc xlink:href="art-20180331.xsd#art_NoncashorPartNoncashAcquisitionFixedAssetsUnderCapitalLeaseObligationsAcquired" xlink:label="loc_art_NoncashorPartNoncashAcquisitionFixedAssetsUnderCapitalLeaseObligationsAcquired_9F3B892E3D9D91EFCC224B1335727142" xlink:type="locator" />
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    <link:label id="lab_us-gaap_InterestPaidNet_B61ECFEC81F058DF0BC34B13357267D1_terseLabel_en-US" xlink:label="lab_us-gaap_InterestPaidNet_B61ECFEC81F058DF0BC34B13357267D1" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Interest paid &#8211; net of amounts capitalized and defeasement costs</link:label>
    <link:label id="lab_us-gaap_InterestPaidNet_B61ECFEC81F058DF0BC34B13357267D1_label_en-US" xlink:label="lab_us-gaap_InterestPaidNet_B61ECFEC81F058DF0BC34B13357267D1" xlink:role="http://www.xbrl.org/2003/role/label" xlink:type="resource" xml:lang="en-US">Interest Paid, Net</link:label>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InterestPaidNet" xlink:label="loc_us-gaap_InterestPaidNet_B61ECFEC81F058DF0BC34B13357267D1" xlink:type="locator" />
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    <link:label id="lab_us-gaap_IncomeTaxesPaidNet_1E2F7FDEB34A4AAD54384B13357219A4_terseLabel_en-US" xlink:label="lab_us-gaap_IncomeTaxesPaidNet_1E2F7FDEB34A4AAD54384B13357219A4" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Income taxes paid &#8211; net of refunds</link:label>
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    <link:label id="lab_art_NoncashorPartNoncashAcquisitionFixedAssetsNotUnderCapitalLeaseObligationsAcquired_9AC1678D7ACD9C2BFA1B4B133572184C_terseLabel_en-US" xlink:label="lab_art_NoncashorPartNoncashAcquisitionFixedAssetsNotUnderCapitalLeaseObligationsAcquired_9AC1678D7ACD9C2BFA1B4B133572184C" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:type="resource" xml:lang="en-US">Acquisition of property, plant, and equipment on accrual</link:label>
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