Removes warning about hidden being undefined (#129)

Why:

* When compiling a view that used this module it would throw a warning
  saying that `hidden/0` was undefined

This change addresses the need by:

* Tricking the compiler to now throw the warning since we know that it
  will be defined as we're only calling it conditionally on the fact that
  it has been overridden.

lib/jsonapi/view.ex

@@ -110,7 +110,8 @@ defmodule JSONAPI.View do
         hidden =
           if Enum.member?(__MODULE__.__info__(:functions), {:hidden, 0}) do
             Deprecation.warn(:hidden)
-            __MODULE__.hidden()
+            this = __MODULE__
+            this.hidden()
           else
             hidden(data)
           end