Fix NoMethodError: protected method around_validation

On Rails 4.2, state_machine is broken due to using `public_send` instead of a plain `send`. This patch is simply a formalized version of pluginaweek#348.

lib/state_machine/integrations/active_model.rb

@@ -513,6 +513,7 @@ def define_validation_hook
         def around_validation(object)
           object.class.state_machines.transitions(object, action, :after => false).perform { yield }
         end
+        public :around_validation
 
         # Creates a new callback in the callback chain, always inserting it
         # before the default Observer callbacks that were created after