Fix exception handling with respect to Rollbar

Makes some minor fixes to get this sytem working:

* Blank out `sinatra.error` when catching a Pliny exception so that
 Rollbar doesn't see it.
* Add Rollbar Sinatra middleware.
* Remove base endpoint error handling block for Pliny errors (everything
 gets sent up to the middleware).
* Use Rollbar's new `disable_monkey_patch` config as written by Pedro.
* Bump Rollbar version to get new config.