Added required -p flag to mkdir for CDH5

In 0.23 HDFS made the -p flag mandatory to silently make parent directories. MrJob was failing on CDH5 because mkdir was no longer working. -p has been in the hdfs client for a long time, so adding it shouldn't break old versions.

JIRA for the change here: https://issues.apache.org/jira/browse/HADOOP-8551