Remove Airbrake dependency

PR to resolve these issues: https://github.com/simplybusiness/rabbit_feed/issues/3 https://github.com/simplybusiness/rabbit_feed/issues/8

@dawid-sklodowski Could you please sign off?
I'm not completely convinced that user of a gem should have exceptions unconsciously reported to Airbrake.

However if we mentioned that in documentation it should be fine.

We could have a documentation section saying that exceptions will go to Airbrake if it is present in host application. Also user can change this behaviour by monkey patching RabbitFeed class in his initialisers like:

```ruby
class RabbitFeed
  def exception_notify(exception)
    Logger.error(exception)
    # or Sentry.notify_exception(exception)
  end
end
```