Use updated Qt formula for Homebrew

The `--HEAD` flag is no longer necessary:

https://github.com/mxcl/homebrew/commit/4bd60168ffbff7246656fcd737cb1f26407afd54
Use Ruby 2.0.0.p353 as default

Resolves reported vulnerability in p247.

https://www.ruby-lang.org/en/news/2013/11/22/heap-overflow-in-floating-p
oint-parsing-cve-2013-4164/
Configure Bundler for parallel gem installs

* Set it globally for OS X.
* Determine number of cores dynamically.
* Pick one number less than number of cores to avoid deadlock errors.
  http://archlever.blogspot.com/2013/09/lies-damned-lies-and-truths-backed-by.html
* Only install `--pre` for Bundler.
* Remove `pg` and `unicorn` gems as they will be installed during `bundle`
  for a Rails project.
Fix homebrew conditional
Add rcm

The rcm suite of tools are used to manage dotfiles. They can symlink
dotfiles from a repo into your homedir, turn an existing configuration
file into a symlinked dotfile, and in general make it easy to try new
dotfile repos.
Use thoughtbot/rcm for Homebrew

Back when rcm was an experiment by Mike Burns, the `homebrew-rcm` GitHub
repo was hosted under the `mike-burns` account. It has since been moved
to `thoughtbot` so that more may share in the love.
Change Homebrew install path

Looks like it's hosted in a different repository now.
Add command for accepting XCode agreement on Mavericks
update homebrew install page