Rather than installing Ruby 2.1 from source using rbenv, it is also possible to install Ruby 2.1 using APT, by using the Brightbox PPA: https://launchpad.net/~brightbox/+archive/ruby-ng
The Phusion Passenger APT packages are compatible with the Brightbox Ruby packages.
This. I'm so grateful to Brightbox for having relieved me from the rvm/rbenv madness. Same painless packages on both development and production. Priceless.