MageStack comes with an auto installer for RVM, which will get you most of the way towards installing Ruby based tools.
root, execute the following,
apt-get install bzip2 rvm_install.sh
After the installation process is complete, you can drop back to an unprivileged user.
www-data, to install Ruby 2.2.4, execute the following,
source .bash_profile rvm install ruby-2.2.4 --autolibs=0
Once installed, you are free to use
gem to install Compass,
gem install compass