Add a Procfile

This allows the app to be run with foreman, as well as things like
Heroku.
Prefer $PORT over config for app's port

Services like Heroku use process.env.PORT to determine which port
the application should be running on.

This shouldn't cause any change for us, because we don't set $PORT
anywhere. We'll fall back to the application config for this value.
Merge pull request #502 from alphagov/add-procfile

Add a Procfile, use $PORT