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