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 see--I thought I tried multiple versions with this paradigm yesterday, but apparently that would only work to get the currently defined alpha image. We can pin it in our previous way until we're happy with...probably beta channel, at some point.