Migrate to Heroku's Cedar stack

Following [the docs](https://devcenter.heroku.com/articles/cedar-migration), I reprovisioned the demo app using Heroku's Cedar stack: http://cedar-demo-activeadmin.herokuapp.com/admin

@gregbell and @pcreux, I set you both up with access to the Heroku app itself

I also set up the free Librato stats dashboard: https://metrics.librato.com/share/dashboards/sxw8synq