Set MIX_ENV to 'prod' on Heroku. Use current heroku CLI commands. Name the app.
Use current versions of rebar and Elixir.
Run Phoenix app on Heroku with recommended production optimizations.

See : https://github.com/phoenixframework/phoenix/blob/master/README.md
Merge pull request #1 from grempe/working-config

Update install and run instructions to achieve working config.