http://apple.stackexchange.com/questions/153790/how-to-fix-brew-after-its-upgrade-to-yosemite
>$ brew
/usr/local/bin/brew: /usr/local/Library/brew.rb: /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby: bad interpreter: No such file or directory
/usr/local/bin/brew: line 23: /usr/local/Library/brew.rb: Undefined error: 0
>$ cd /usr/local/Library
>$ ls
Aliases/ Contributions/ ENV/ Formula/ Homebrew/ LinkedKegs/ Taps/ brew.rb*
>$ git pull origin master
...bunch of stuff...
Then you can run 'brew update'.