Add tests for access draft travel-advice editions

Also fixed 404 handling when edition is missing.
Add Timecop for some upcoming time dependent tests
Add change metadata fields to travel-advice

Also changes the updated_at for these artefacts to use the published_at field if set.
Update to new gemfury source URL format

See http://blog.gemfury.com/post/43472005936/unleash-the-fury-io
Merge pull request #77 from alphagov/travel_advice_change_metadata

Add travel-advice change metadata fields to artefacts.