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.