Replaced GirlFriday async handler by Celluloid

This is an opinionated proposal to replace actor pattern library used in async handler GirlFriday by Celluloid.

Celluloid is a popular and under active development actor pattern library with many features like Futures, Supervisors, Timers and Finite State Machine.

This pull request change Rollbar's default async handler from a GirlFriday::WorkQueue to a Celluloid::Future to process block of code asynchronously. Both are conceptually pretty close, async handler is executed on another thread for concurrency.
Celluloid::Future.new uses threads from Celluloid internal thread pool ([link](https://github.com/celluloid/celluloid/wiki/Futures)).