Remove potentially unnecessary steps

Command Line Tools ships either LLVM or Clang/LLVM. Certain libraries
(ImageMagick maybe?) require GCC. Installing GCC 4.2 via Homebrew dupes
allows the developer to compile any library that requires non-LLVM GCC.
However, it isn't clear which libraries we use actually rquire non-LLVM
GCC.

http://stackoverflow.com/a/5708732/303882
http://robots.thoughtbot.com/post/27985816073/the-hitchhikers-guide-to-riding-a-mountain-lion

Permission fixing used to be necessary but Homebrew fixed this in this
commit:

https://github.com/mxcl/homebrew/commit/8aa4f9dd3bbd885350776171d3d9403d381e1a59

Certain system libraries may have been necessary for Ruby 1.9.3 when we
were on RVM, which enforced certain requirements:

https://github.com/wayneeseguin/rvm/blob/master/scripts/functions/requirements/osx_brew#L93

The only way to know whether these dependencies are all still necessary
is to remove them all and try the script, adding them back one-by-one if
necessary.
Update docs, stick to zshrc