Fallback to Config if Mix.Config does not exist, even if Config does not

exist either

For some weird reason Code.ensure_loaded?(Config) is false on OTP
releases even though `use Config` from config files works just fine.

priv/templates/sample_config.eex

@@ -3,7 +3,7 @@
 # NOTE: This file should not be committed to a repo or otherwise made public
 # without removing sensitive information.
 
-<%= if Code.ensure_loaded?(Config) do
+<%= if Code.ensure_loaded?(Config) or not Code.ensure_loaded?(Mix.Config) do
   "import Config"
 else
   "use Mix.Config"