Fix the deprecation warnings

[quentin-bettoum]

Fixed the deprecation warnings by following the recommandations.

warning: use Mix.Config is deprecated. Use the Config module instead
  config/config.exs:3

Compiling 1 file (.ex)
warning: ^^^ is deprecated. It is typically used as xor but it has the wrong precedence, use Bitwise.bxor/2 instead
  lib/xxhash.ex:21:27

warning: ^^^ is deprecated. It is typically used as xor but it has the wrong precedence, use Bitwise.bxor/2 instead
  lib/xxhash.ex:46:27

The tests pass

[ttvd]

Hi Quentin, thanks for the fix! I will merge it asap, when I get a chance.