With Nitrous.io the documentation states that other Ruby version managers may be used instead of RVM.
However, sudo is needed for installing rbenv
Is RVM currently the only option on nitrous.io?
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.