Use class_attribute and not config_attribute.

Also require the needed files.

activerecord/lib/active_record/store.rb

@@ -1,3 +1,6 @@
+require 'active_support/concern'
+require 'active_support/core_ext/class/attribute'
+
 module ActiveRecord
   # Store gives you a thin wrapper around serialize for the purpose of storing hashes in a single column.
   # It's like a simple key/value store backed into your record when you don't care about being able to
@@ -38,7 +41,7 @@ module Store
     extend ActiveSupport::Concern
 
     included do
-      config_attribute :stored_attributes
+      class_attribute :stored_attributes
       self.stored_attributes = {}
     end