The original Rubocop configuration was copied from other thoughtbot
projects. Of course, Rubocop comes with its own set of default settings.
Therefore, we don't need to re-specify settings that it already has --
we only need to specify the ones that we're overriding.
Besides this, Rubocop isn't even in the Gemfile, so even though Hound
runs on GitHub automatically, it'd be nice to spot style issues as you
are editing files.
Given this, this commit simplifies the Rubocop configuration and also
adds Rubocop to the Gemfile so that you can configure your editor to
lint files as you are editing them.
Elliot Winkler