Fix - keepalive ping for heroku needs to match httpd.coffee

Please see this discussion that @technicalpickles started.
        https://github.com/github/hubot/commit/0fbcf74#commitcomment-3077952

Looks like I introduced a very serious bug for folks running hubot on heroku in commit https://github.com/github/hubot/commit/0fbcf74.

httpd.coffee was updated in an earlier commit. The corresponding change to robot.coffee was missed. We now ping `#{robot.name}/ping` instead of `hubot/ping`

Instead, I think it would be a fair call to simply `git revert 0fbcf74` and go back to how it was. httpd.coffee consists purely of admin stuff.