allow newer ruby-libvirt (#158)

As per #158, vagrant-libvirt depends on ruby-libvirt transitively through fog.  Since fog/fog#2706 was resolved, fog now requires `ruby-libvirt ~> 0.5.0`, therefore vagrant-libvirt should allow both 0.4.x and 0.5.x.  Furthermore, according to semantic versioning, we currently have no good reason to reject anything newer below 1.x.  If such a restriction is required in the future (e.g. `< 0.7.0`) then it is likely that fog's `.gemspec` will take care of that on our behalf anyway.

Closes #158.