feat(*): provision using CoreOS beta channel

With CoreOS 494.1.0 including Docker 1.3.2 and landing in their beta channel, Deis may be able to formally begin tracking beta now. This change just reworks the Vagrantfile to track beta and be more similar to the upstream coreos-vagrant version, updates provider IDs to use CoreOS beta 494.1.0, and specifies that as our minimum requirement.