Enable warning_as_errors for tests

mix.exs

@@ -8,12 +8,7 @@ def project 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,
+      elixirc_options: [warnings_as_errors: true],
       start_permanent: Mix.env() == :prod,
       aliases: aliases(),
       deps: deps(),