Add Airbrake dependency with the minimal configuration

This is so that we can get the application to post exceptions through
to Errbit.
This is no longer necessary since https://github.com/airbrake/airbrake/pull/268 was merged (airbrake 3.1.17 and later).

As per the [instructions](https://github.com/alphagov/wiki/wiki/Configuring-an-app-to-use-Errbit), this should probably be a placeholder with just a comment...