Add updated_at field to Json output.

This will be needed by the search indexing process to be able to
do some kind of version compairson - to be able to ensire that an older
version doesn't overwrite a newer versions (e.g while doing bulk index
rebuilds)
Update docs to reflect updated_at field
Merge pull request #18 from alphagov/add_updated_at_to_output

Add updated_at field to Json output.