Heroku deploys fail with hard coded port

Heroku passes in a dynamic port to this project under `process.env.PORT`. When it's hard coded, the Heroku port (I believe) is ignored, and then Heroku tries to bind to the process under the port it expects, which nothing is running on, then eventually fails with

``` prolog
Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch
heroku[web.1]: Stopping process with SIGKILL
heroku[web.1]: State changed from starting to crashed
heroku[web.1]: Process exited with status 137
```

Due to this commit:

https://github.com/erikras/react-redux-universal-hot-example/commit/3ec5ecb4a949c76d06da91d8a7688e8feefb3f48#diff-b9cfc7f2cdf78a7f4b91a753d10865a2

Ping @trungtin and @quicksnap for merging https://github.com/erikras/react-redux-universal-hot-example/pull/966

This took me about 6 hours to debug :) Suggest rolling back change ASAP. I suspect many heroku deploys will fail with the latest version of this library using a hard coded port.