Use ruby_build instead of RVM

This is a big change and I welcome discussion whether this is a good idea. I tried to update the RVM version installed by the RVM cookbook and I ran into trouble. Upon closer inspection it seems the RVM cookbook has been [abandoned](http://community.opscode.com/cookbooks/rvm) (last updated Feb 2012). On the other hand, ruby_build is [more active](http://community.opscode.com/cookbooks/ruby_build) (last updated May 2013). Using this PR we install Ruby 1.9.3-p448 as `/usr/local/bin/ruby`.

Advantages:
- cleaner Vagrantfile and gitlab site-cookbook;
- not being stuck with an outdated RVM cookbook.

Disadvantages:
- makes it harder to switch rubies: you would have to change the ruby_version attribute in `site-cookbooks/gitlab/attributes/vagrant.rb` and run `vagrant provision`;
- initial `vagrant up` takes longer because RVM was downloading a binary whereas ruby_build compiles from source.