Add option to use sucker_punch as async handler

As suggested in girl_friday's README:

> Please note: I recommend using Brandon Hilkert's sucker_punch library in preference to girl_friday now. Please consider using it for future projects.

Thus, I implemented the sucker_punch support for rollbar-gem, and also add testing spec for that.

But one thing I would like to discuss about is the configuration API of setting sucker_punch as async handler. Originally, I thought it's a replacement of GirlFriday so I made `use_async` detect the existence of sucker_punch and then set sucker_punch as async handler.

However, since the syntax of sucker_punch jobs is more like Sidekiq than GirlFriday (e.g. need to add a job class to process payloads), so I finally changed my mind to add a `use_sucker_punch` option instead.

Any opinion or feedbacks? We really enjoy Rollbar, and we hope we can make it better :smile: