Update PROCFILE and externalize port as environment variable

Updated PROCFILE to call app.js, instead of index.js.  Also, externalized port as enviornment variable in order to deploy this app to Heroku properly.

Here is the link to the deployed app on Heroku:
https://germ-tracker.herokuapp.com/

@jjmerino, @kmeurer, @jamesongamble, @suprbh - please review and merge this PR.