Allow specifying rubygems package

This diff looks bigger than it is, sorry. I tidied up some whitespace and other lint things while I was making the changes.

The main change allows a user of the module to specify the package to be used for rubygems. As included in the docs this is important if you want to install a 1.9 series ruby on Ubuntu. Without it installing the rubygems package will bring in the ruby1.8 package, replacing the 1.9 one.