Make the version of rubygems configurable

This lets you set desired version of rubygems regardless of which one is installed by the brightbox package.

Slightly off-topic: I used the new [chef-dk](http://www.getchef.com/downloads/chef-dk/) and there was an error trying to run `berks`, due to the hack in the metadata.rb file where it uses `Chef::Version`. The chef's version in chef-dk is actually 11.14.0.alpha.1 and cannot be parsed by `Chef::Version`. So I had to comment out all those attribute declarations to get specs to run (and add another spec). Regardless, that junk in metadata.rb should be removed, I will submit a separate pull request for that.