Add an .env file. This files contains the local VARS that are different in your local setting from the heroku environment. However, if everything runs fine, you can just ignore the warning, or do touch .env.
You can also differentiate by creating a Procfile.local file and run heroku local -f Procfile.local, to force using a local start script.
I just read: https://devcenter.heroku.com/articles/heroku-local. That explains it all.
For a working example: http://github.com/halcwb/GenUnitApp.git. When you have the heroku toolbelt installed you can heroku local -f Procfile.bash on mac or linux, or heroku -f Procfile.local for windows, and go to http://localhost:3000, to check the result.