Update PROCFILE and externalize port as environment variable

Updated PROCFILE to use app.js, instead of index.js.  Also, updated server.js so that it uses environment variable as port if exist.  Otherwise, it uses 4568 as default port#.  With these changes, we can deploy our app to Heroku.  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.