Added CentOS support.

The yum variable $releasever is different in RHEL and CentOS and the yum
repos only have a path for the RHEL variable. There is a bugzilla ticket
to fix, https://bugzilla.redhat.com/show_bug.cgi?id=1144568, but until
its completed this patch is needed.
Merge pull request #1 from mego22github/centos_support

Added CentOS support.