fixes #3855 - pin ruby-libvirt to 0.4 for Ruby 1.8 workaround

This is temporary due to https://www.redhat.com/archives/libvir-list/2013-December/msg00593.html, I intend to revert the commit once Chris has released 0.5.1.  But for now, it's causing problems in Jenkins.

Please let Jenkins run through and make sure Ruby 1.8 passes.