Version bump to 2.9.3

Includes:

* Make sure`robot.pingIntervalId` is kept after setting up Heroku ping  interval cc https://github.com/github/hubot/pull/795 @DexterV
* Allow `robot.respond` to work when there is preceding whitespace cc https://github.com/github/hubot/pull/794 @ahayworth 
* Update `robot.parseHelp` to be synchronous, so it's easier to test cc https://github.com/github/hubot/pull/762 @bouzuya 
* Reduce Heroku ping interval from 20 minutes to 5 minutes to keep hubot from going unavailable cc https://github.com/github/hubot/pull/809