Fix bug where mkdir_on_hdfs not work correctly on Hadoop 2.2 / 0.23.
use mrjob.compat.version_gte for comparing version.
Modify _mkdir_on_hdfs. Add unittest.
fix unittest
Merge pull request #845 from ShusenLiu/master

Fix bug where mkdir_on_hdfs not work correctly on Hadoop 2.2 / 0.23.