I have and Ubuntu 12.04 LTS, running Ruby 1.9.3 and rails 3.2.8 and I wat to upgrade to ruby 2.1.1 and the latest stables for everything else.
I already had RVM installed, so updated it, installed ruby 2.1.1, updated ruby gems and rails
rvm get stable
rvm install 2.1.1 --with-openssl-dir=$HOME/.rvm/usr
rvm use 2.1.1 --create --default
gem update
gem install rails