Cc/test/6/pin elixir version to last known good

[chriscrabtree]

Hi, first, I appreciate your work on this project! I was starting down a related path, so I was grateful to find your project.

Adding the package to a project revealed some sensitivities to specific versions of Elixir. I confirmed this by running the test suite and noticing issue #6 as well.

This simple PR adds the version info necessary for users of asdf to successfully run the test suite.

My goal with this PR was to establish a working baseline with all tests passing, so that I could then bump to a newer version of Elixir and hopefully help get the project working for those too.