README updated; Thoughtbot sold Airbrake in 2011.

This change removes reference to Thoughtbot providing support for Airbrake.

> *February 7, 2012*
> “Late last year, Airbrake was acquired by the team at Exceptional, inc.”
> — http://robots.thoughtbot.com/post/17212734809/airbrake-acquired-by-exceptional

Additionally, and anecdotally, Airbrake is less worthy of recommendation compared to when this README was written. In fact a common reason now for switching from Airbrake to Errbit is that the former cannot be relied upon to report errors.