define AR callback as method for Rails 4.1 compatibility

rails/rails#12981

This was causing me issues when trying to destroy an instance of a model with a protect block defined.

lib/protector/adapters/active_record/base.rb

@@ -9,6 +9,8 @@ module Base
           include Protector::DSL::Base
           include Protector::DSL::Entry
 
+          before_save :before_save_callback
+
           # We need this to make sure no ActiveRecord classes managed
           # to cache db scheme and create corresponding methods since
           # we want to modify the way they get created
@@ -25,11 +27,6 @@ module Base
             end
           end
 
-          before_destroy do
-            return true unless protector_subject?
-            destroyable?
-          end
-
           # Drops {Protector::DSL::Meta::Box} cache when subject changes
           def restrict!(*args)
             @protector_meta = nil
@@ -133,6 +130,12 @@ def destroyable?
         def can?(action, field=false)
           protector_meta.can?(action, field)
         end
+
+        private
+        def before_save_callback
+          return true unless protector_subject?
+          destroyable?
+          end
       end
     end
   end