Run on Elixir 1.15 and Erlang 26

[dabaer]

To be merged at the same time as

• https://github.com/exercism/elixir-test-runner/pull/108
• https://github.com/exercism/elixir-analyzer/pull/386

[angelikatyborska]

I'm glad at least repo didn't cause any trouble 😁 thank you! I'm not clicking the "accept" button yet to signal that it shouldn't be merged yet, but no changes are required here.

[dabaer]

I'm glad at least repo didn't cause any trouble 😁 thank you! I'm not clicking the "accept" button yet to signal that it shouldn't be merged yet, but no changes are required here.

I breathed a sigh of relief when it "just worked"