feat: detect RCs

[gigatim]

This modification allows for use of release candidates of elixir.

[TheArrowsmith]

I'd love to see this merged! Am currently trying, and failing, to deploy an app with (recently released) Elixir v1.17.0.rc-0.

[gigatim]

I'd love to see this merged!

Try the buildpack from gigalixir: https://github.com/gigalixir/gigalixir-buildpack-elixir

Check the gigalixir documentation on how to specify your buildpack.

[TheArrowsmith]

Oh hello Tim, I didn't notice that this PR was from Gigalixir! 😁

For anyone else reading this: I managed to upgrade my Gigalixir app to 1.17.0-rc.0 by adding a .buildpacks file that looks like this:

https://github.com/gigalixir/gigalixir-buildpack-elixir
https://github.com/gigalixir/gigalixir-buildpack-phoenix-static
https://github.com/gigalixir/gigalixir-buildpack-releases.git

as well as changing the version to 1.17.0-rc.0 in the usual places.