add formula for rabbitmq 3.0.4, used by Heroku/Bigwig

Based on [the 3.0.4 formula in mainline (3bffe56)](https://github.com/Homebrew/homebrew/blob/3bffe5629cbf6b308ec045621339214b468fee2d/Library/Formula/rabbitmq.rb), only added a `conflicts_with 'rabbitmq'` and changed the class name.