If you do not wish to use RVM which is pre-installed then you can install rbenv. Run the following commands within your Nitrous.IO console:
$ git clone https://github.com/sstephenson/rbenv.git ~/.rbenv
$ echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bashrc
$ echo 'eval "$(rbenv init -)"' >> ~/.bashrc
$ git clone git://github.com/sstephenson/ruby-build.git ~/.rbenv/plugins/ruby-build
$ source ~/.bashrc
These steps can be found within the Octopress guide which utilizes rbenv on Nitrous.IO.