deflaker

DecisionOldOldTreeBuilderTest::randomForestTest failed for us on commit 18b60b59: ``` randomForestTest on randomForestTest(quickml.supervised.tree.decisionTree.DecisionOldOldTreeBuilderTest)(quickml.supervised.tree.decisionTree.DecisionOldOldTreeBuilderTest) Time elapsed: 5.291 sec

`DecisionOldOldTreeBuilderTest.mockTreeTest` fails for us on commit `2b348d5` but did not fail on [the corresponding run on Travis](https://travis-ci.org/sanity/quickml/builds/157995433). Our tool confirms this test to be flaky. The stack trace is below....