Update heroku commands

[Homerade]

Heroku now requires a flag and the heroku project you're adding things too. Without the flag you get the following

$ heroku buildpacks:set https://github.com/HashNuke/heroku-buildpack-elixir
 ›   Error: Missing required flag:
 ›     -a, --app APP  app to run command against
 ›   See more help with --help

I tried to push a PR with the following small changes to the readme but got a 403

# Set and add the buildpacks for your Heroku app
heroku buildpacks:set https://github.com/HashNuke/heroku-buildpack-elixir -a mysterious-meadow-6277
heroku buildpacks:add https://github.com/gjaldon/heroku-buildpack-phoenix-static -a mysterious-meadow-6277