Support box_version option

Vagrant has supported [box versioning](https://docs.vagrantup.com/v2/boxes/versioning.html) since v1.5. Although it doesn't seem to be well documented, it's actually possible to set `box_url` to a [metadata file](https://docs.vagrantup.com/v2/boxes/format.html) rather than a specific box. This PR makes kitchen-vagant support Vagrant's [box_version](http://docs.vagrantup.com/v2/vagrantfile/machine_settings.html) option, without which, Vagrant will default to using the latest. 

This will be very useful to teams who have a large number of proprietary Chef cookbook repositories. Now instead of having to update the `.kitchen.yml` in every repo for every new base box update, it'll be possible to do global updates through a single metadata file hosted on a server.