Support for :rubygem_version option to #rvm helper

In our Chef configuration, we use the chef-rvm cookbook and lock in a specific rvm version and rubies.

    e.g.
    node.default[:rvm][:version] = '1.22.13'
    ..
    node.default['rvm']['rubies'] << 'ruby-1.8.7-p374'
   
Re-provisioning nodes some time later, things broke with the error below because rvm was installing rubies with the latest rubygems version. The chef-rvm cookbook now supports rubygems_version (https://github.com/fnichol/chef-rvm/pull/100), but without this patch, there is no way to pass that through to the initial 'rvm install' command. Recipes will eventually fail due to the locked rvm version.

> There is no checksum for 'http://production.cf.rubygems.org/rubygems/rubygems-2.0.14.tgz' or 'rubygems-2.0.14.tgz', it's not possible to validate it.
This could be because your RVM install's list of versions is out of date. You may want to
update your list of rubies by running 'rvm get stable' and try again.
If that does not resolve the issue and you wish to continue with unverified download
add '--verify-downloads 1' after the command.
>
> There has been an error while trying to fetch rubygems. 
Halting the installation.