Fix Elixir compiler warning

Author: lpgauth

mix.exs

@@ -9,7 +9,7 @@ defmodule Uuid.Mixfile do
      version: "2.0.7",
      language: :erlang,
      erlc_options: [
-       {:d, :erlang.list_to_atom('ERLANG_OTP_VERSION_' ++ :erlang.system_info(:otp_release))},
+       {:d, :erlang.list_to_atom(~c"ERLANG_OTP_VERSION_" ++ :erlang.system_info(:otp_release))},
        :deterministic,
        :debug_info,
        :warn_export_vars,