Use IUS Community CentOS repository if not Redhat platform

IUS Community does separate builds for CentOS to Redhat
as they build against the latest Redhat minor release that
CentOS may not have currently released

An example is currently RHEL 6.6 is released, but CentOS 6.6
is not. A RHEL build against 6.6 may link to a package that
doesn't exist, or has a library version bump, causing
the package to not be able to be installed on CentOS.