Update yum sources for CentOS 6.x

The ruby-rgen package has moved from http://yum.puppetlabs.com/el/6/products to http://yum.puppetlabs.com/el/6/dependencies. The result of this is that the installation of puppet fails when it cannot locate ruby-rgen.

This PR fixes the issue by adding the new repository to Yum for the affected CentOS 6.x definitions.