Bring back compat with ActiveModel::Serializers

The current AMS re-write (which will debut as v0.9.0) has changed how
serializers are resovled, and removed the ArraySerializerSupport module.
This change will ensure Draper properly integrates with both AMS
pre-0.9.0, and the future.

lib/draper/railtie.rb

@@ -45,7 +45,9 @@ module Draper
 
     initializer "draper.setup_active_model_serializers" do |app|
       ActiveSupport.on_load :active_model_serializers do
-        Draper::CollectionDecorator.send :include, ActiveModel::ArraySerializerSupport
+        if defined?(ActiveModel::ArraySerializerSupport)
+          Draper::CollectionDecorator.send :include, ActiveModel::ArraySerializerSupport
+        end
       end
     end
 

spec/dummy/spec/decorators/active_model_serializers_spec.rb

@@ -4,8 +4,13 @@ describe Draper::CollectionDecorator do
   describe "#active_model_serializer" do
     it "returns ActiveModel::ArraySerializer" do
       collection_decorator = Draper::CollectionDecorator.new([])
+      if defined?(ActiveModel::ArraySerializerSupport)
+        collection_serializer = collection_decorator.active_model_serializer
+      else
+        collection_serializer = ActiveModel::Serializer.serializer_for(collection_decorator)
+      end
 
-      expect(collection_decorator.active_model_serializer).to be ActiveModel::ArraySerializer
+      expect(collection_serializer).to be ActiveModel::ArraySerializer
     end
   end
 end