Issue
Review apps require us to manually start a worker dyno. Is there something we can add to either Procfile or app.json?
Resolution
You can specify which, how many, and what size dynos to scale with formation in your app.json. It sounds like you'll want a web and worker entry.
More info can be found in Heroku's Dev Center: https://devcenter.heroku.com/articles/app-json-schema#formation