Setup publish to hex through CI

[dvcrn]

While migrating from travis to github ci, I forgot the auto-publish to hex.

Add a new CI step to publish to hex when there is a new tag

[dvcrn]

I've setup a hex.publish step based on https://github.com/marketplace/actions/publish-to-hex that should trigger when new tags are published.

I'll keep this issue open until we can test it with the next release