fix(config): fix issue where `config.ip` is undefined in non-production environments

This is somewhat of a small issue discovered by @chair9design: https://github.com/DaftMonk/generator-angular-fullstack/issues/205#issuecomment-43713129.
`config.ip` is only defined in the production 'env' config file, so when running the server in any other environment (specifically development), no ip is passed to either `app.listen` or  `console.log`(within server.js).

From what i can tell, passing a null hostname to app.listen (which calls [server.listen](http://nodejs.org/api/http.html#http_server_listen_port_hostname_backlog_callback) ) is not an issue. However, since we are directly using this config variable in server.js, it seems like we should default it to something.