Add delayed_job_attempt_threshold config value

This allows to set a threshold for Honeybadger notifications for Delayed Job.

Oftentimes it's useful to let DJ retry the jobs before polluting HB with notifications i.e. third party web services can time-out the first time, but succeed eventually. There is nothing a developer can do about it and it does not make sense to notify them unless it fails N consecutive times.

Let me know if the approach is correct and if I need to add anything to the documentation.