Investigate the frequent failures of Travis CI

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Running org.apache.tajo.engine.planner.physical.TestExternalSortExec
Run: testLeftOuterJoinWithNull1[3]
Run: testLeftOuterJoinWithNull2[3]
Run: testLeftOuterJoinWithNull3[3]
Run: testRightOuterJoinWithThetaJoinConditionInWhere[3]
Run: testLeftOuterWithEmptyTable[3]
Run: testFullOuterJoinPredicationCaseByCase1[3]
Run: testRightOuterJoin1[3]
Run: testJoinOnMultipleDatabases[3]
Run: testFullOuterJoinWithEmptyTable1[3]
Run: testFullOuterJoinWithEmptyIntermediateData[3]
Run: testCrossJoinWithThetaJoinConditionInWhere[3]
Run: testComplexJoinCondition1[3]
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 23.036 sec - in org.apache.tajo.engine.planner.physical.TestExternalSortExec
Running org.apache.tajo.engine.planner.physical.TestTupleSorter
Run: testComplexJoinCondition2[3]
Run: testComplexJoinCondition3[3]
Run: testComplexJoinCondition4[3]
Run: testComplexJoinCondition5[3]
Run: testComplexJoinCondition6[3]
2015-04-05 17:31:25,478 WARN: org.apache.tajo.engine.planner.global.GlobalPlanner (visitTableSubQuery(1536)) - Can't find left most SubQuery in the UnionNode.
Run: testComplexJoinCondition7[3]
2015-04-05 17:31:27,707 WARN: org.apache.tajo.engine.planner.global.GlobalPlanner (visitTableSubQuery(1536)) - Can't find left most SubQuery in the UnionNode.
Run: testDifferentTypesJoinCondition[3]
Run: testComplexJoinsWithCaseWhen[3]
Run: testJoinWithOrPredicates[3]
Run: testLeftOuterJoinWithEmptyTable1[3]
Run: testLeftOuterJoinWithEmptyTable2[3]
Run: testLeftOuterJoinWithEmptyTable3[3]
Run: testLeftOuterJoinWithEmptyTable4[3]
Run: testLeftOuterJoinWithEmptyTable5[3]
Run: testJoinCoReferredEvals1[3]
Run: testCrossJoinWithEmptyTable1[3]
Run: testLeftOuterJoinWithThetaJoinConditionInWhere[3]
Run: testJoinWithJson2[3]
Run: testOuterJoinAndCaseWhen1[3]
Run: testJoinOnMultipleDatabasesWithJson[3]
Killed
Run: testInnerJoinWithEmptyTable[3]
Run: testJoinWithDifferentShuffleKey[3]
Run: testJoinFilterOfRowPreservedTable1[3]
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Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 6.096 sec - in org.apache.tajo.engine.planner.physical.TestFullOuterHashJoinExec
Running org.apache.tajo.engine.planner.physical.TestFullOuterMergeJoinExec
2015-04-04 10:10:38,960 WARN: org.apache.tajo.engine.planner.global.GlobalPlanner (visitTableSubQuery(1536)) - Can't find left most SubQuery in the UnionNode.
Tests run: 6, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 10.963 sec - in org.apache.tajo.engine.planner.physical.TestFullOuterMergeJoinExec
Running org.apache.tajo.engine.planner.physical.TestHashSemiJoinExec
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 1.06 sec - in org.apache.tajo.engine.planner.physical.TestHashSemiJoinExec
Running org.apache.tajo.engine.planner.physical.TestMergeJoinExec
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 1.037 sec - in org.apache.tajo.engine.planner.physical.TestMergeJoinExec
Running org.apache.tajo.engine.planner.physical.TestExternalSortExec
3000000 rows (52 MB)
Killed
Tests run: 316, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 814.466 sec - in org.apache.tajo.engine.query.TestJoinQuery
Running org.apache.tajo.engine.planner.physical.TestTupleSorter
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 37.473 sec - in org.apache.tajo.engine.planner.physical.TestTupleSorter
Running org.apache.tajo.engine.planner.physical.TestLeftOuterHashJoinExec
Tests run: 5, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 6.753 sec - in org.apache.tajo.engine.planner.physical.TestLeftOuterHashJoinExec
Running org.apache.tajo.engine.planner.physical.TestHashJoinExec
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