Elixir 1.15+: Fix call to Kernel.self/0

[mpaulmier]

See https://github.com/elixir-lang/elixir/blob/v1.15/CHANGELOG.md#elixir-6

[Kernel] Raise instead of warning on undefined variables. Previously, an undefined variable would attempt to invoke a function of the same name, which led to confusing error messages, especially to newcomers. To enable the previous behaviour, invoke Code.compiler_options(on_undefined_variable: :warn) at the top of your mix.exs

[Shakadak]

Approved