fix isup to handle http://domain.com case

When we use hubot with irc adaptor and slack, slack automatically inject http:// in front of urls. In this case, isup does not work properly. This PR is to fix it.
I would make the http a non capturing group. That way, you don't have to change the `msg.match`, ie: `/is (?:http\:\/\/)?(.*?) (up|down)(\?)?/i`