ruby cart: Explicitly req. ruby-rdoc dep for rhel

Bug 1066246
https://bugzilla.redhat.com/show_bug.cgi?id=1066246

Although `ruby-rdoc` gets pulled in by `rubygems`, it's commonly needed
or desired for typical ruby usage and so should be required here as well.
Merge pull request #5097 from dobbymoodge/BZ1066246

Merged by openshift-bot