fix port binding error when deploy to heroku

I tried to deploy mean to heroku but return R10 (Boot timeout) error,
then I found [this article](http://www.vibesphere.com/2013/06/gotchas-for-node-js-apps-on-heroku/) meet the same error and explain the reason of R10.

> when on Heroku, port will be exported to an environment variable and available as process.env.PORT

So I guess it would be better use process.env.PORT as default to make mean work fine after deploy to heroku.