Rake tasks which run tests fail when the tests fail

:fork_and_knife:

The original motivation for this PR was that `rake spec:all` would never fail. That rake task is now a composite of the `spec`, `spec:javascripts`, and `spec:rubocop` tasks. The first one of those to fail will cause `spec:all` to fail.

Note that Travis goes back to running each task individually. This is a small step to make the most of our Travis runs. In practice, it means that we'll know if both RSpec and Rubocop are failing after one Travis run.

Finally, we needed a small bit of Rubocop love in light of the recent Rubocop upgrade.