:next_version method doesn't return anything for the most recent version

[batter]

Say you have a model named Document which declares has_paper_trail.

> d = Document.create!
=>  #<Document id: 1, name: nil>
> d.update_attribute(:name, 'Ben')
=> true
> d.update_attribute(:name, 'Andy')
=> true
> d.previous_version
=> #<Document id: 1, name: "Ben">
> d.previous_version.next_version
=> nil # should return the live version (#<Document id: 1, name: "Andy">)

The next_version method is currently broken, in that if it is called on the latest reified version of a model prior to the live model, it returns nil when it should actually return the live model.