disconnected - adding support for ruby scl

Okay this is my first attempt to enable SCL on RHEL6 for katello-utils package (I do not want to start with our heavyweight katello.spec).

How it works: In our RHEL6 buildroot on our koji we have ruby193-build macros installed, therefore SCL works there. But not for Fedora - we do not want to use SCL on Fedoras. Therefore this patch adds support of SCL for RHEL6 only.

As you can see it builds for both:

http://koji.katello.org/koji/taskinfo?taskID=26510

http://koji.katello.org/koji/taskinfo?taskID=26509

Comments, opinions? If there are no objections, I will do the same for katello itself.