Support for :rubygem_version option to #rvm helper

Patch to support installing a rvm ruby with a specified :rubygems_version.

See: https://github.com/fnichol/chef-rvm/pull/252

In the context of provisioning, it's desirable to lock in specific rvm version, rubies, and rubygem versions. Needed this patch to avoid the error below when doing so.

> 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This could be because your RVM install's list of versions is out of date.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.