Make www['url'] mandatory.

WIP.
This could be replaced with ``self.www['url'].rstrip('/')``, but I still don't understand what you're trying to achieve in this part of the code.

If you want to enforce that the port specified in the www config is the same as in the url, this is plain wrong, as if buildbot is being proxied (take nginx for instance), we could perfectly end up on a different port than the one buildbot is running.

If this is what you're trying to achieve, that whole ``if`` block can disapear.