Beef up Vagrantfile

This PR adds dynamically defined Vagrant boxes with the name of the form `<language>-<release>` (e.g., `ruby-precise`, `android-trusty`).

README includes instructions on how to create the Vagrant box with Packer for use with this Vagrantfile. One problem here is that the Bento fork is mine (at the moment) and we do not officially support it. Mentioning the fork in this document may give a wrong impression.
Well, the `vm_templates` is actually in this repo, so we could just drop this conditional.