add ActiveSupport.on_load announcement

lib/active_model_serializers.rb

@@ -57,3 +57,5 @@ begin
 rescue LoadError => ex
   # rails on installed, continuing
 end
+
+ActiveSupport.run_load_hooks(:active_model_serializers, ActiveModel::Serializer)

test/serializer_test.rb

@@ -814,4 +814,12 @@ class SerializerTest < ActiveModel::TestCase
       ]
     }, actual)
   end
+
+  def test_active_support_on_load_hooks_fired
+    loaded = nil
+    ActiveSupport.on_load(:active_model_serializers) do
+      loaded = self
+    end
+    assert_equal ActiveModel::Serializer, loaded
+  end
 end