Use method to return json_library instead of a module attribute

lib/ueberauth/strategy/twitter.ex

@@ -10,30 +10,6 @@ defmodule Ueberauth.Strategy.Twitter do
   alias Ueberauth.Auth.Extra
   alias Ueberauth.Strategy.Twitter
 
-  @json_library Application.get_env(:ueberauth, :json_library, Ueberauth.Config.json_library())
-  unless Code.ensure_loaded?(@json_library) do
-    IO.puts("""
-    The json_library '#{inspect(@json_library)}' does not appear
-    to be available.  A json library is required
-    for Ueberauth to operate. Is it configured as a
-    dependency in mix.exs?
-    In config.exs your expicit or implicit configuration is:
-      config ueberauth,
-        json_library: #{inspect(@json_library)}
-    In mix.exs you will need something like:
-      def deps() do
-        [
-          ...
-          {:#{String.downcase(inspect(@json_library))}, :version}
-        ]
-      end
-    """)
-
-    raise ArgumentError,
-          "Json library #{String.downcase(inspect(@json_library))} does " <>
-            "not appear to be a dependency"
-  end
-
   @doc """
   Handles initial request for Twitter authentication.
   """
@@ -129,13 +105,13 @@ defmodule Ueberauth.Strategy.Twitter do
       {:ok, %{status_code: 401, body: _, headers: _}} ->
         set_errors!(conn, [error("token", "unauthorized")])
       {:ok, %{status_code: status_code, body: body, headers: _}} when status_code in 200..399 ->
-        body = @json_library.decode!(body)
+        body = json_library().decode!(body)
 
         conn
         |> put_private(:twitter_token, token)
         |> put_private(:twitter_user, body)
       {:ok, %{status_code: _, body: body, headers: _}} ->
-        body = @json_library.decode!(body)
+        body = json_library().decode!(body)
         error = List.first(body["errors"])
         set_errors!(conn, [error("token", error["message"])])
     end
@@ -148,4 +124,8 @@ defmodule Ueberauth.Strategy.Twitter do
     |> options
     |> Keyword.get(key, default)
   end
+
+  defp json_library do
+    Application.get_env(:ueberauth, :json_library)
+  end
 end