Add organisation to Publishing API payload

Using an array means the format, which will be shared with specialist-publisher
and then used by manuals-frontend, can accommodate multiple publishing
organisations. This isn't currently a requirement, but Specialist Publisher
will need it at some point and doing it this way reduces churn in something
defined across multiple apps.
Merge pull request #52 from alphagov/add_organisation_to_publishing_api_payload

Add organisation to Publishing API payload