Fix random Argument error when update note

In some strange cases Ruby thinks `system` is not AR field but
Kernel.system call and raises exception during save. It should prevent
this random wierd behaviour

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/note.rb

@@ -502,6 +502,6 @@ class Note < ActiveRecord::Base
   end
   def editable?
-    !system
+    !read_attribute(:system)
   end
 end