box_add should enable the usage of --box-version #175

Under vagrant box add --help there's an option to be able to define which version of the box we want to download:

--box-version VERSION Constrain version of the added box

Currently, there's no way to define a specific version and make it run with that one, it's only the latest or having this hardcoded under the VagrantFile.

The request is to extend the box_add() method to accept this optional parameter