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
Fix "Could not verify the SSL certificate" errors

The error message indicates the connection failed because OpenSSL was
unable to verify the server certificate. The SSL certificates that comes
with Mac OS X 10.8 may be outdated.

http://bit.ly/ruby-ssl

This also should help avoid critical OpenSSL errors:

https://github.com/Homebrew/homebrew/commit/dc763ad98c4c5d1ce72b85e5b2ea90313fad92b2
http://arstechnica.com/security/2014/04/critical-crypto-bug-in-openssl-opens-two-thirds-of-the-web-to-eavesdropping/
Build "linking OpenSSL" change from 44a5266
Use command -v to test command existence

Use preferred POSIX `command -v` test for command existence instead of
previous `which` technique. `which` exits with status `1` if nothing is
found. That would cause the script to abort because we use `set -e`.

http://stackoverflow.com/questions/592620/how-to-check-if-a-program-exists-from-a-bash-script
Make mac script run idempotently

Use preferred POSIX `command -v` test for command existence instead of
previous `which` technique. `which` exits with status `1` if nothing is
found. That would cause the script to abort because we use `set -e`.

http://stackoverflow.com/questions/592620/how-to-check-if-a-program-exists-from-a-bash-script

Remove GCC dependency in mac script

GCC is no longer necessary to compile Ruby. It was fixed in Ruby about
two years ago. LLVM has been supported since 1.9.3-p125.

https://bugs.ruby-lang.org/issues/5076
http://stackoverflow.com/questions/8032824/cant-install-ruby-under-lion-with-rvm-gcc-issues
Merge branch 'master' of https://github.com/thoughtbot/laptop