Setting up MRI 1.9.3 on OSX Lion

First uninstall Xcode:

$ sudo /Developer/Library/uninstall-devtools --mode=all 

Download Apple's official command line tools: https://daw.apple.com/cgi-bin/WebObjects/DSAuthWeb.woa/wa/login?appIdKey=d4f7d769c2abecc664d0dadfed6a67f943442b5e9c87524d4587a95773750cea&path=%2F%2Fdownloads%2Findex.action

$ brew update
$ brew install autoconf automake
$ brew install https://raw.github.com/Homebrew/homebrew-dupes/master/apple-gcc42.rb
$ rvm get head
$ rvm install 1.9.3-head