Switch from ActiveSupport's on_load to railtie initializer.

This resolves the insistent warning about errors in transactions, covered in #867.

lib/thinking_sphinx/railtie.rb

@@ -1,6 +1,8 @@
 class ThinkingSphinx::Railtie < Rails::Railtie
-  ActiveSupport.on_load :active_record do
-    include ThinkingSphinx::ActiveRecord::Base
+  initializer 'thinking_sphinx.initialisation' do
+    if defined?(ActiveRecord::Base)
+      ActiveRecord::Base.send :include, ThinkingSphinx::ActiveRecord::Base
+    end
   end
 
   rake_tasks do