After a couple of hours of not getting "/usr/bin/env: ruby: No such file or directory" error and google searching, I uninstalled my system ruby 1.8.7 (I also had 1.9.3 installed via rvm) by doing sudo apt-get purge ruby rubygems.
I am trying to run ruby on my RPi (Raspbian/Debian distro).
I have installed ruby with apt-get command, but it has installed 1.9. 3 version. I wanted newer one, so I removed that one and next, according to some guide, I have installed ruby-install and used it to install ruby 2.1. 5. After some downloading, compiling it has installed ruby to my ~/. Rubies/ruby-2.1.