Linting rule only for dev and test

Production build broke because our custom lint rule was compiled. The
credo linter is not available and not necessary in production.

Solution: create separate directory for dev tools.

app/mix.exs

@@ -32,7 +32,8 @@ defmodule Firehose.MixProject do
   end
 
   # Specifies which paths to compile per environment.
-  defp elixirc_paths(:test), do: ["lib", "test/support"]
+  defp elixirc_paths(:test), do: ["lib", "lib_dev", "test/support"]
+  defp elixirc_paths(:dev), do: ["lib", "lib_dev"]
   defp elixirc_paths(_), do: ["lib"]
 
   # Specifies your project dependencies.