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
Install gems after configuring bundler for parallel installation.
Remove linux-prereq script by switching to Bash

Required changes:

* Change #!
* Move pre-req actions directly into linux
* Switch uses of $+command for which (zsh-ism)
* Don't source ~/.zshrc to pickup new exports
* Add setting of zsh as $SHELL
* Update README

Ancillary changes:

* Switch [ with [[ (best practice)
* Switch if-newline-then with if; then (style)
* Use $( ) and $(( )) (best practice)
* Switch wget with curl (consistency)
change thoughtbot/rcm to thoughtbot/formulae

Started reading the thoughtbot playbook last week, which led me to laptop. Calling mac from Terminal went fine untill the last installation which yielded "ERROR: no available formula for rcm". Googling the error message brought me to Matthew Sumner's commit # 2915cf1f696777a7438d0bbf92f688f32e1e8264 in the dotfile repo. Applying the same modif to the laptop mac shell script got rid of the error message. If you accept my pull-in request, that'd be my first ofishal contribution to the open source world.
Update mac file to tap from thoughtbot/formulae

In reference to [this pull](https://github.com/thoughtbot/dotfiles/pull/222)
Linux: Add nodejs for rails asset pipeline rendering
Linux: Add sqlite3-dev and sqlite3 cli
add own customisations