Added missing dependencies.

When using the recipe on a new ubuntu 12.04 machine through vagrant, the recipe didn't installed the rubies and rvm complained about these three missing packages: pkg-config, libgdbm-dev and libffi-dev. 
I've added them and tested it on a new machine.
The change was based on this pull request: https://github.com/fnichol/chef-rvm/pull/153