Require Paperclip::Schema only when used with AR

This will solve an issue when using Paperclip without Active Record
such as Mongoid.

Fixes #919

lib/paperclip/glue.rb

@@ -8,15 +8,9 @@ module Paperclip
       base.extend ClassMethods
       base.send :include, Callbacks
       base.send :include, Validators
-      base.send :include, Schema
+      base.send :include, Schema if defined? ActiveRecord
       base.class_attribute :attachment_definitions
 
-      if defined?(ActiveRecord)
-        ActiveRecord::ConnectionAdapters::AbstractAdapter.send(:include, Paperclip::Schema)
-        ActiveRecord::ConnectionAdapters::Table.send(:include, Paperclip::Schema)
-        ActiveRecord::ConnectionAdapters::TableDefinition.send(:include, Paperclip::Schema)
-      end
-
       locale_path = Dir.glob(File.dirname(__FILE__) + "/locales/*.{rb,yml}")
       I18n.load_path += locale_path unless I18n.load_path.include?(locale_path)
     end