change_host_name is a capability in Vagrant 1.2

Vagrant 1.2.x introduces a capability system for guests. The `change_host_name` method no longer exists.

See the changelog at https://github.com/mitchellh/vagrant/blob/master/CHANGELOG.md#120-april-16-2013. I'm not sure why this isn't mentioned as a backwards incompatibility (@mitchellh?).