Heroku compile issue (Elixir Buildpaack) ** (Mix) Expected :version to be a SemVer version
could not compile dependency jsonex, mix compile failed. You can recompile this dependency with `mix deps.c> ** (Mix) Expected :version to be a SemVer version
The issue was in the Jsonex mix.exs: the project version was "2.0". Forking and changing the version to "2.0.0"
resolves the issue.
Note that if you change dependencies in a project, you may need also to ensure the the mix.lock file changes too: the safest way is to delete the mix.lock file - mix deps.clean doesn't necessarily do it, if the change is subtle.