Fix nil need_ids on imported artefacts

some artefacts were imported before
we deployed the change to add need_ids
to Artefacts. hence the [earlier migration](https://github.com/alphagov/panopticon/blob/master/db/migrate/20140324144244_copy_artefact_need_id_to_array_field_need_ids.rb) didn't
account for those artefacts. this migration should
fix it and the related errors [in errbit](https://errbit.production.alphagov.co.uk/apps/52f0ceeb0da1152f0b000003/problems/535e833d0da1151271000d33).

[find_and_modify docs](http://docs.mongodb.org/v2.2/reference/method/db.collection.findAndModify/)