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 nefore poluting 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.
Merge pull request #93 from alphasights/dj_thresshold

Add delayed_job_attempt_threshold config value