Backoff for when reporting issues occur

[bagedevimo]

During the recent rollbar outage (Apr 22, 06:10 PDT), we noticed our Delayed::Job queue was filling up with jobs to report errors to Rollbar, which would then fail and raise a new job, per this code:

https://github.com/rollbar/rollbar-gem/blob/b3dcbb4f437a48c73fbfe791a1ae67f67d4e4b1b/lib/rollbar/notifier.rb#L220-L263

It'd be great if this had some kind of back-off.