The solution to this on my system was to change the order of the buildpacks.
Unless https://github.com/HashNuke/heroku-buildpack-elixir.git is FIRST bad things seem to happen like those in the thread above.
To move them around. use:
heroku buildpacks:remove URL
heroku buildpacks:add URL
For example
$ heroku buildpacks
=== my_app_name Buildpack URLs
1. https://github.com/HashNuke/heroku-buildpack-elixir.git
2. https://github.com/gjaldon/phoenix-static-buildpack