BUGFIX Versioned_Version->relField()

Copied from DataObject, since we can't use the $fallback opion in this case
(will try to retrieve from wrong class). Specifically, this was needed
to generate a GridField view which lists Versioned_Version records with the Author.Email field.
Since I can't (easily) overload Versioned_Version (its instanciated within Versioned),
I don't see an easy way to access this relationship otherwise. Ideally Versioned_Version would extend DataObject (or behave as one), but without requiring a table, but that's a much larger refactoring.