Feature #3882 Use ruby-libvirt's latest version.

0.5.1 is out and it compiles against 1.8.7 and 1.9.3p327 perfectly, so I'm getting rid of the temporary fix here.

https://www.redhat.com/archives/libvir-list/2013-December/msg00846.html

- http://projects.theforeman.org/issues/3882