When trying to compile Opus using Elixir 1.4, then we get:
could not compile dependency :ex_doc, "mix compile" failed.
You can recompile this dependency with "mix deps.compile ex_doc",
update it with "mix deps.update ex_doc" or clean it with "mix deps.clean ex_doc"
Since Opus is using Elixir ~>1.4, it should keep the ex_doc version at 0.18.0 as mentioned in the documentation (ref: https://github.com/elixir-lang/ex_doc#using-exdoc-with-mix)
When trying to compile Opus using Elixir 1.4, then we get: