Bump xgboost from 1.1.1 to 1.2.0 (#299)

Bumps [xgboost](https://github.com/dmlc/xgboost) from 1.1.1 to 1.2.0.
- [Release notes](https://github.com/dmlc/xgboost/releases)
- [Changelog](https://github.com/dmlc/xgboost/blob/master/NEWS.md)
- [Commits](dmlc/xgboost@v1.1.1...v1.2.0)

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requirements-test.txt

@@ -1,6 +1,6 @@
 # Supported models
 scikit-learn==0.23.2
-xgboost==1.1.1
+xgboost==1.2.0
 lightgbm==2.3.1
 statsmodels==0.11.1
 git+git://github.com/scikit-learn-contrib/lightning.git@782c18c12961e509099ae84c68dd361010642f7e