Revert "Xgboost version workaround"

This reverts commit e06b2e4.

Author: InvalidPointer

tox.ini

@@ -54,8 +54,7 @@ deps =
     tests: pandas
     tests: swat
     tests: kerberos ; platform_system != "Windows"
-    tests: xgboost == 0.82; python_version < '3.5'
-    tests: xgboost; python_version >= '3.5'
+    tests: xgboost == 0.82
     tests: lightgbm
     codecov: codecov >= 1.4.0
     doc: sphinx == 1.8