XGboostRegressorBostonHousingNumericTest is failing

The numeric test for xgboost regressor converter is failing with the following:

```
FAIL: test_boston_housing_simple_random_forest_regression (coremltools.test.test_boosted_trees_regression_numeric.XGboostRegressorBostonHousingNumericTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/zaccharieramzi/workspace/coremltools/coremltools/test/test_boosted_trees_regression_numeric.py", line 223, in test_boston_housing_simple_random_forest_regression
self._train_convert_evaluate_assert(subsample = 0.5)
File "/Users/zaccharieramzi/workspace/coremltools/coremltools/test/test_boosted_trees_regression_numeric.py", line 217, in _train_convert_evaluate_assert
self._check_metrics(metrics, bt_params)
File "/Users/zaccharieramzi/workspace/coremltools/coremltools/test/test_boosted_trees_regression_numeric.py", line 195, in _check_metrics
msg = 'Failed case %s. Results %s' % (params, metrics))
AssertionError: 0.016184903142875712 != 0 within 0.0001 delta (0.016184903142875712 difference) : Failed case {}. Results {'samples': 506, 'rmse': 0.016184903142875712, 'max_error': 0.2887819620530898}
```

I am on mac OS 10.14.