Pik Managing Different Rails Versions Troubles
How to set up and config changes you need to make, more info in this gist.
First of all, RTFM!
Then, one thing that might go by unnoticed while you install anything, is your gem version. For instance, to install rails 3-1, you HAVE to have RubyGems version 1.8.11 or newer.
To update, simply do:
$ gem update --system
Also, you need isolate, so:
$ gem install isolate
One more thing to consider, after you install and update stuff, you need to...unfinished thought 💸