Fix error logging in object_changes_deserialized

I was trying to debug some code that was throwing a:

```
Psych::DisallowedClass: Tried to load unspecified class: Time
```

This warning was not being printed, because `Psych::DisallowedClass` is a subclass of `Psych::Exception`. So I think you want to use `is_a?`, not `instance_of?`.

lib/paper_trail/version_concern.rb

@@ -355,7 +355,7 @@ def object_changes_deserialized
         begin
           PaperTrail.serializer.load(object_changes)
         rescue StandardError => e
-          if defined?(::Psych::Exception) && e.instance_of?(::Psych::Exception)
+          if defined?(::Psych::Exception) && e.is_a?(::Psych::Exception)
             ::Kernel.warn format(E_YAML_PERMITTED_CLASSES, e)
           end
           {}