make mix tasks work correctly (MIX_ENV=prod)

[mergefailure]

• for example :

MIX_ENV=prod mix deps    
** (Code.LoadError) could not load ask_bunny/config/prod.exs
    (elixir) lib/code.ex:1193: Code.find_file/2
    (elixir) lib/code.ex:758: Code.eval_file/2
    (mix) lib/mix/config.ex:158: anonymous fn/2 in Mix.Config.__import__!/2
    (elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
    (mix) lib/mix/config.ex:157: Mix.Config.__import__!/2
    (stdlib) erl_eval.erl:680: :erl_eval.do_apply/6
    (elixir) lib/code.ex:240: Code.eval_string/3