Removed disabling SELinux prereq; no longer needed

This SELinux problem was caused by the yumpkg.py module that used the
yum Python interface for CentOS 6 machines. That was removed in favor of
the yumpkg.py that shells out to yum which does not have this problem.
This workaround can be removed.
Merge pull request #13 from saltstack-formulas/remove-selinux-prereq

Removed disabling SELinux prereq; no longer needed