Automatically install Ruby version if missing

On recent projects, we've been making the Ruby setup experience better by
checking `.ruby-version` and automatically installing the Ruby version if it's
not on the machine.

This applies only to rbenv and ruby-build users, which are the majority of
thoughtbot folks. It errors out with a friendly message for chruby or other
non-rbenv users.

Our previous Bundler idempotency code also did not work if you had a version
of Bundler installed on a different Ruby version. This commit fixes that use
case.

All of this makes the situation better when a teammate upgrades from Ruby
2.1.2 to Ruby 2.1.3 or from Ruby 2.1.3 to Ruby 2.1.4 and you don't have that
version on your machine. All you have to do at that point is `./bin/setup`
again, just like any other "my machine is not set up properly for this
project" problem.
I'm using `rbenv` and `ruby-build` from Homebrew and I don't have this directory.