Update all ES queries to use Elasticsearch DSL (bug 1036168)

Not just to be explicit. This was one place where the serializers didn't like the new format returned by the search query. Previously it just took the dict returned from the search query and stuck it on `latest_version`. Using elasticsearch_dsl it's no longer a dict but a `Result` object with attributes and it didn't know how to deal with that. But explicit is better here anyway. And yeah, for such a small nested dict I didn't think it warranted a new serializer.