HEROKU_URL -> HEROKU_GIT_URL

This change is necessary since when we're using hipchat adapter heroku
free plan, we need HEROKU_URL=http://appname.heroku.com configured.
(ref
https://github.com/hipchat/hubot-hipchat#quickstart-hubot-for-hipchat-on
-heroku)
So we need to name the git url something other then HEROKU_URL.