Hack in a fix for robot.messageRoom()

I'm not actually a big fan of this change, but the underlying idea here is to support Hubot's `robot.messageRoom()`.  Slack's adapter currently doesn't let us override the room, which ends up being undefined in that case.  At the moment we have to use room ID's instead of room names, which isn't super fun.  I looked around a bit and couldn't find any API docs, so maybe there is some way to use a room name here that I'm not aware of.

As I said, I don't love this code but it's the only way I could make the feature work.  This is basically a feature request in the form of a pull request.