vi /usr/local/Library/brew.rb
Change line 1 from:
#!/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
to:
#!/System/Library/Frameworks/Ruby.framework/Versions/Current/usr/bin/ruby -W0
Save and exit
cd $(brew –repository) && git commit -a -m 'new ruby version 2.0'