Use the `port` value provided in config.json to build the web hook URL

app.initializeHooks does not take bot.options.port into account when it builds the GitHub web hook URL. For example, if bot.options.port === 5000 and bot.options.url === 'http://foo.com', the hook url will be http://foo.com?repo_fullname=user/reponame. This patch uses bot.options.port, only if a port is not already part of bot.options.url.