Switch Airbrake config file from YAML to Ruby

This means that we can load the config directly, rather than having
to load it as YAML and set the configuration again when we want to
use it.
Add a new rake task to notify Errbit of a deploy

Airbrake includes three rake tasks, one of which does deploy notification:

    https://github.com/airbrake/airbrake/blob/v3.1.15/lib/airbrake/shared_tasks.rb

However, that task only loads configuration from Rails initializers, so doesn't
work for Rack apps unless Airbrake's configuration is already loaded somehow.
Since the other two tasks from the Airbrake gem aren't useful to us here
either, we're no longer including them in the Rakefile (one is Heroku-specific
and the other, to test the setup, is even more Railsy than the deploy one).

Instead, we've added a new rake task to notify Errbit of a deploy, which
is very similar to Airbrake's built-in one but loads our config instead
of looking for a Rails initializer.