Vagrantfile adapted to work with Vagrant 1.1+

Hi folks!

I was trying to run puppet-sandbox with the latest Vagrant release (1.1) and it wasn't working because it introduces some different configuration objects (http://docs.vagrantup.com/v2/vagrantfile/version.html).

I adapted the Vagrantfile to work with Vagrant 1.1+ but obviously I broke the backward compatibility (<=1.0) wich in fact, it was already broken.

Looking forward to hearing from you.