Removed need for Heroku to re-compile the Monit binary on each push

This PR implements a basic caching mechanism on the build/compilation assets so that subsequent Heroku pushes build super-fast. If Monit is upgraded the slug compiler detects the change and pulls down the latest code automatically as per the previous process.

Also includes a minor upgrade to Monit 5.9. Feedback from #2 included for a much cleaner PR.