Reduce logic

Require roaster_id in form and if missing use Rails method
find_or_create_by when record does not exist yet
Remove now unused class method
Require roaster_id field
Utilize FactoryGirl association functionality

Docs at:
https://github.com/thoughtbot/factory_girl/blob/master/GETTING_STARTED.m
d#associations
Test Review.aggregate_rating method
Update + add roasts controller specs