Bumped CDH5 to 5.2.0 and Mesos to 0.21.0

Update to readme

Bumped Mesos version

Bumped CDH5 Version to 2.5.0-cdh5.2.0 & Mesos 0.21.0

Revert "Bumped CDH5 Version to 2.5.0-cdh5.2.0 & Mesos 0.21.0"

This reverts commit 769ec67e1a106afffb76cf172076ac765736a41f.