Update heroku cli command in README
Update heroku cli command in README

In the Heroku Toolbelt they changed config:add in config:set

yuku-t/heroku-buildpack-elixir,fcruxen/heroku-buildpack-elixir,camshaft/heroku-buildpack-elixir,bbhoss/cloudfoundry-buildpack-elixir,danielberkompas/heroku-buildpack-elixir,teodor-pripoae/heroku-buildpack-elixir,hectorip/buildpack-elixir-phoenix,crigor/buildpack-elixir-mix-env,HashNuke/heroku-buildpack-elixir,lbarratt/heroku-buildpack-elixir,gogolok/cloudfoundry-buildpack-elixir,aerosol/heroku-buildpack-elixir,cloud8421/heroku-buildpack-elixir