Backport support for Vagrant pre-releases into 1.3.x

Would it be possible to backport this (#100) into 1.3 series?

I *can* live without, but as I now constantly run development version of Vagrant, I also have to use locally patched vagrant-berkshelf plugin. And as I all the time uninstall and install the plugins, it's easy to forget to use the local one. =)