Squashed 'subtree/rabit/' changes from 18f4d6c..3bf8661
Squashed 'subtree/rabit/' changes from 18f4d6c..3bf8661

3bf8661 add std before basic

git-subtree-dir: subtree/rabit
git-subtree-split: 3bf8661ec1109aecd3630c401d98612e41471e7b

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