Fetch deps based on MIX_ENV instead of prod
Fetch deps based on MIX_ENV instead of prod

HashNuke/heroku-buildpack-elixir,fcruxen/heroku-buildpack-elixir,yuku-t/heroku-buildpack-elixir,aerosol/heroku-buildpack-elixir,camshaft/heroku-buildpack-elixir,hectorip/buildpack-elixir-phoenix,crigor/buildpack-elixir-mix-env,lbarratt/heroku-buildpack-elixir,danielberkompas/heroku-buildpack-elixir