Improves test message prefixes outside of manishearth/ChatExchange Travis environment

It now links to the build in the current repo, even if that's not the manishearth/ChatExchange. (Using the right ID but the wrong repo name, as it previously could do when run from my repo, produced a buggy-looking Travis page.)

It also uses ChatExchange@localhost, linked to the git repo, when not being run on Travis. (I wasn't sure what label to use here. It was previously producing a 404 link.)