New latest changes endpoint

https://www.pivotaltracker.com/story/show/81091510
Amendments after PR comments
Moved specialist-sector requests to single endpoint
* Moved all requests to a single endpoint returning both the sector
content and the latest changes.
* Changes timestamp to public_updated_at for latest changes as standard
Altered latest changes link to include full URI
Merge pull request #11 from alphagov/latest-endpoint

New latest changes endpoint

https://www.pivotaltracker.com/story/show/81091510