Retrieve the most recent Unpublishing

Some documents have been unpublished more than once. We want to make
sure we retrieve the most recent Unpublishing instance, otherwise we may
not perform the correct action (i.e. the first unpublishing may be
because the document was "published in error" whilst the most recent is
a redirect)
Merge pull request #1616 from alphagov/most-recent-unpublishing

Retrieve the most recent Unpublishing