# update rvm
rvm get stable

# see already installed versions of ruby
rvm list

# see what versions of ruby are available for download
rvm list known

# pick one and install, e.g.
rvm install 2.1[.1]

# use and set as default
rvm use 2.1[.1] --default

# install/update rails (the -V shows the process, otherwise it looks as if stuck)
gem install rails -V

Q: What’s the difference between version 2.1.[.1] and 2.1-head?

A: The head version would be more up-to-date thus might end up being more unstable.