Ability to install a vagrant plugin with a specific version and source

Hi there, I was very excited to try the release candidate of berkshelf for vagrant-1.1
Here are my patches to be able to install berkshelf-1.3.0.rc1 with either one of those command-lines:

    vagrant plugin install --gem-source http://rubygems.org/ --gem-version 1.3.0.rc1 berkshelf
    vagrant plugin install --gem-source http://rubygems.org/ --gem-pre berkshelf

Note that for some reason, if I leave http://gems.hashicorp.com in the list of repositories, rubygems cannot locate berkshelf-1.3.0.rc1.