mix: add ex_const dependency

mix.exs

@@ -150,6 +150,7 @@ defmodule Pleroma.Mixfile do
       {:benchee, "~> 1.0"},
       {:esshd, "~> 0.1.0", runtime: Application.get_env(:esshd, :enabled, false)},
       {:ex_rated, "~> 1.3"},
+      {:ex_const, "~> 0.2"},
       {:plug_static_index_html, "~> 1.0.0"},
       {:excoveralls, "~> 0.11.1", only: :test},
       {:mox, "~> 0.5", only: :test}