Register autoload for ActiveRecord::LogSubscriber

Before f56b418 this constant was being
loaded in the railtie, but it was removed since we didn't needed it
anymore.

It causes some application to break if they try to reference the
subscriber before `ActiveRecord::Base` is loaded.

Author: rafaelfranca

activerecord/lib/active_record.rb

@@ -50,6 +50,7 @@ module ActiveRecord
   autoload :Inheritance
   autoload :Integration
   autoload :InternalMetadata
+  autoload :LogSubscriber
   autoload :Migration
   autoload :Migrator, "active_record/migration"
   autoload :ModelSchema