rvm is the Ruby Version Manager, manages different version of ruby running on the same server.
If you get this error put this line into your .bashrc
$ source ~/.rvm/scripts/rvm
$ rvm use system
Now using system ruby.
Ruby gives you a clue. Something to do with the green
if doornumber < 100 || doorcolour == green
puts "That's the post delivered to the wrong house pat!"
It's missing the double quotes around the green, it should read doorcolour == "green"
If you see this error it's evident it cannot find the header files, which generally means it's can't find the files on your system or they are not installed, more likely they are not installed. You can install the ruby header files by running:
# yum install ruby-devel