Treat warnings as errors outside of tests.

mix.exs

@@ -8,6 +8,11 @@ defmodule Pleroma.Mixfile do
       elixir: "~> 1.4",
       elixirc_paths: elixirc_paths(Mix.env()),
       compilers: [:phoenix, :gettext] ++ Mix.compilers(),
+      elixirc_options: if Mix.env() == :test do
+        []
+      else
+        [warnings_as_errors: true]
+      end,
       start_permanent: Mix.env() == :prod,
       aliases: aliases(),
       deps: deps(),