Refactor #airbrake_current_user

Fix for https://github.com/airbrake/airbrake/issues/313 based on the (excellent) work by @tonyarkles and his PR https://github.com/airbrake/airbrake/pull/331

- ensure we return the DB connection   
- split the DB interaction from populating the hash

The reason I prefer doing it this way is that it isolates the problematic spot into a single short method.
I prefer to leave it like this. The previous code had checks to ensure the returned value was a hash but it felt awkward to me. I prefer to let the `airbrake_current_user` either return the hash as expected or nil.