README updated; Thoughtbot sold Airbrake in 2011.

“Late last year, Airbrake was acquired by the team at Exceptional, inc.”
— http://robots.thoughtbot.com/post/17212734809/airbrake-acquired-by-exceptional (February 7, 2012)
Merge pull request #485 from pda/patch-1

README updated; Thoughtbot sold Airbrake in 2011.