Updates for elixir v0.12.5

Catching up to some deprecations.

[is_regex -> Regex.regex?/1] one is changed as similar way as "lib/mix/test/mix/tasks/new_test.exs" in the following commit.
https://github.com/elixir-lang/elixir/commit/2cc4fd2eb5f5a70c4dafd5a72eb4fd0d1c399a41