Display the name of the 'Last updated by' user

- it displays 'Last updated... by [Person]' when the person is *not* the system user (Version.public_user0

- see 'Last updated ... by Toby' (in attachment, lower right of page)
![screen_shot_last_update](https://cloud.githubusercontent.com/assets/5799/4886165/fb662cea-6379-11e4-8843-017b5cb728f8.png)

This should be `describe '#last_update'`