Elixir 1.15

[mnishiguchi]

https://github.com/nerves-project/nerves/issues/873

• seems like CI won't get triggered for this project...
• ✅ mix deps.update --all locally
• ✅ mix format locally
• ✅ mix test locally
• ✅ mix dialyzer locally

[fhunleth]

CircleCI had the build forked PRs option off. I changed that for next time. Thanks!