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
Update Homebrew even if already installed
Install Suspenders instead of Rails

We more typically create Rails apps using Suspenders. Rails is a
dependency of Suspenders, so it will still be installed.
Replace all uses of echo with printf

The echo builtin is very unportable and should be avoided. More
information can be found here:

http://wiki.bash-hackers.org/commands/builtin/echo
Use Homebrew for `hub` on OS X

The previous method places the `hub` command in ~/bin/hub, which shows
up in the dotfiles repo for thoughtbot/dotfiles users. We could
gitignore the `bin/hub` gem there, but the hub README recommends using
Homebrew or a package manager when it is available.

This leaves the Linux version functionally the same.
Upgrade Ruby to 2.1.1
Fix test broken by ddb17283d3
Start Postgres after it is installed

* Set up plist so Postgres will start up at login.
* Create file for other potential `brew services start` lines. Redis?
Allow personal customizations via ~/.laptop.local

Use same `.local` convention as https://github.com/thoughtbot/dotfiles.
Test all officially supported linux distros

Includes more thorough rails-level tests and docs for the
aforementioned.
Fix how we source personal additions to laptop

If the file ~/.laptop.local doesn't exist, the laptop script returns a
failing exit code. This doesn't really make sense, and gets in the way of our
test runner.  Wrap this in slightly more verbose `if` to ensure a
successful exit code.
Don't hardcode Ruby version

* Use http://ruby.thoughtbot.com/latest endpoint.
* We'll update the file when necessary, which is stored on S3 and served
  via Fastly.
* Apply style guideline: "Use snake_case for variable names and ALLCAPS
  for environment variables."
  https://github.com/thoughtbot/guides/tree/master/style#shell