use ActiveSupport.on_load :active_record instead of an initializer

(cherry picked from commit 67865aac3227bd9c515052f6fbee446b4e724971)

lib/paperclip/railtie.rb

@@ -4,7 +4,7 @@ module Paperclip
   if defined? Rails::Railtie
     require 'rails'
     class Railtie < Rails::Railtie
-      initializer "paperclip.insert_into_active_record" do
+      ActiveSupport.on_load :active_record do
         Paperclip::Railtie.insert
       end
       rake_tasks do