Heroku Guide needs a blurb about setting the elixir version

[zachmccormick]

Environment

• Elixir version (elixir -v):
• Phoenix version (mix deps):
• NodeJS version (node -v):
• NPM version (npm -v):
• Operating system:

Expected behavior

Following the guide will work

Actual behavior

Need to add an elixir_buildpack.config file with the line elixir_version=1.7.4 or similar in it. Following the directions as they stand today will result in an error such as

supervisors expect the child to be a module, a {module, arg} tuple or a map with the child specification

on deploy

[edwinthinks]

I can confirm that this is occurring. I suggested to bump the default version of elixir to at least contain the bug fix related to what you encountered. I have a PR open here: HashNuke/heroku-buildpack-elixir#140 which I think would eliminate the requirement (maybe?) to include a blurb on it?

I think either or both would make sense :)