If someone chose RVM to work with Ruby I suggest you will need to type:
'echo "source \$HOME/.rvm/scripts/rvm" >> ~/.bashrc'
Because otherwise your terminal will remember this source only for current session and next time you ask for 'ruby -v' or 'gem install ...' you'll get answer like 'The program 'gem' is currently not installed. You can install'
Nice guide, by the way