Install Ruby via package manager rather than RVM

Installing Ruby via package manager because rvm caused too much trouble for us. It is also a suspect for the still pertinent git push error (see https://github.com/gitlabhq/gitlab-public-wiki/wiki/Trouble-Shooting-Guide).

If at any time we need to build the latest Ruby version we should start using the https://github.com/fnichol/chef-ruby_build cookbook