Fix ordering in search of manually added historical reports

AAIB have published some non-imported documents which are now appearing at the
top of the finder. This script, when run on production, will re-order them by
updating the `last_update` field which rummager uses to sort them in search.
Merge pull request #365 from alphagov/fix-aaib-report-last-updated-in-rummager

Fix ordering in search of manually added historical reports