Stop rvm from complaining about world writable /usr/local/bin

# Error message
#
/Users/velvetpd/.rvm/gems/ruby-1.9.2-p0@babylon/gems/bundler-1.0.7/lib/bundler.rb:199: warning: Insecure world writable dir /usr/local in PATH, mode 040777

# Fix
# This was probably caused by instructions to install Homebrew
# http://objectliteral.blogspot.com/2010/12/installing-homebrew-on-osx.html
#
chmod o-w /usr/local/bin
chmod o-w /usr/local