Update deprecated #scoped calls

Model#scoped is now deprecated. Model#where(nil) in Rails 4 returns the same result as Model#scoped in Rails 3

app/controllers/api/variants_controller.rb

@@ -58,7 +58,7 @@ module Api
                      @product.variants_including_master
                    end
       else
-        variants = Spree::Variant.scoped
+        variants = Spree::Variant.where(nil)
         if current_api_user.has_spree_role?("admin")
           unless params[:show_deleted]
             variants = Spree::Variant.active