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
Upgrade Ruby to 2.1.0
Remove initdb, fixes #182

The latest Homebrew formula runs this as part of post-install.
Improve idempotency in the linux laptop script

These changes allow the laptop script to run more than once and can be
tested through ./test/runner.sh.  The only significant side effects are:

* Gems (pg, rails, unicorn) will be updated
* ruby $RUBY_VERSION will be set as the global ruby if it wasn't already
* rubygems proper will be updated to the latest release
* hub will be updated
* the heroku toolbelt will be updated.
Update license to 2014
Use correct thoughtbot/homebrew-formulae repo