Fix #3397: early_stop callback does not maximize metric of form NDCG@n- (#3685)
Fix #3397: early_stop callback does not maximize metric of form NDCG@n- (#3685)

* Fix #3397: early_stop callback does not maximize metric of form NDCG@n-

Early stopping callback makes splits with '-' letter, which interferes
with metrics of form NDCG@n-. As a result, XGBoost tries to minimize
NDCG@n-, where it should be maximized instead.

Fix. Specify maxsplit=1.

* Python 2.x compatibility fix

dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost