Update ProductionHeroku.md

Added some text to show tailing of a heroku log; added commentary to clarify that Procfile is not actually required by Heroku.

There is another point on Heroku involving setting the build pack (see http://stackoverflow.com/questions/11510256/deploy-a-play-2-0-2-application-on-heroku ):
heroku config:add BUILDPACK_URL=https://github.com/heroku/heroku-buildpack-scala.git --app app_name

...but I am not sure this is required in all scenarios.