Allow XGBRanker sklearn interface to use other xgboost ranking objectives (#3848)
Allow XGBRanker sklearn interface to use other xgboost ranking objectives (#3848)


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