Check for hex archive file names in order
Check for hex archive file names in order

* First check for file name in $hex_source if specified
* Then check for first hex file in the new hex-<version>.ez format
* Fall back to old hex.ez file (very unlikely to hit this condition)
lbarratt/heroku-buildpack-elixir,HashNuke/heroku-buildpack-elixir,hectorip/buildpack-elixir-phoenix,crigor/buildpack-elixir-mix-env