ref(Vagrantfile): align with upstream coreos-vagrant

See "deis" fork at https://github.com/deis/coreos-vagrant and our diff with upstream at https://github.com/deis/coreos-vagrant/compare/coreos:master...deis
I'm fairly certain this JSON only ever has the latest release, so this would break for users whenever we pin to release which is not the latest (until we update the Vagrantfile). See https://github.com/coreos/coreos-vagrant/issues/83#issuecomment-44874895