Sort AAIB reports by Date of Occurrence before publishing

The initial import imported all of these documents in a pseudo-random
order, meaning that there was no meaningful logic to it when we sorted
them by `updated_at`.

This makes sure that we publish them in the correct order, which
resolves this problem.
Merge pull request #329 from alphagov/publish-aaib-reports-in-correct-order

Sort AAIB reports by Date of Occurrence before publishing