removed airbrake_notify defaults

Not sure if I'm doing it right, but, I don't want to see the deprecation message
`DEPRECATION WARNING: airbrake_notify is deprecated. Please use the Rapns.reflect API instead.` every time. I want to see it only if i'm using/setting airbrake_notify method in the configuration.

When airbrake_notify is set, then we'll get the deprecation warning, otherwise not.