Features/refactored audio encoding

This goes in tandem with Audiobox PR https://github.com/SCPR/Audiobox/pull/1 

* Adds combust to enable rspec testing
* Extracts logic to find the proper campaign out of controller and into the model
* AudioEncoding model now uses STI. There are now two encoding subclasses, AACAudioEncoding and MP3AudioEncoding
* Stream_key column is now unnecessary and removed since audio encoding lookup is now based on campaign_id and the stored encoding options. 
* Is_master column is added to determine if an audio file is a master file