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.