Heroku ping url is HEROKU_URL, not HUBOT_HEROKU_URL

Seems like the docs report the wrong environment variable to set to get Hubot to ping itself and keep your heroku dyno from going idle. Docs current report it as `HUBOT_HEROKU_URL`, looks like `robot.coffee` is using `HEROKU_URL` See: https://github.com/github/hubot/blob/master/src/robot.coffee#L240 for reference.