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.
Unit tests did not run on Ruby 2.2.0-preview1 without this. It is an actual dependency but used to be bundled with the interpreter but as this will change, might as well make it explicit.