Pin xgboost dependency version to 0.90 (#4965)

* Sets xgboost dependency to be 0.90, preventing
   segfaults during TVM python unit tests execution

 * This is discussed in issue #4953

python/setup.py

@@ -164,7 +164,7 @@ setup(name='tvm',
                                'matplotlib'],
                       'extra_feature': ['tornado',
                                         'psutil',
-                                        'xgboost',
+                                        'xgboost==0.90',
                                         'mypy',
                                         'orderedset',
                                         'antlr4-python3-runtime']},