Bust ContentAPI cache when requesting live tags.

This prevents Whitehall endpoints using old data
when triggered by Panopticon immediately after
publishing a tag.
Merge pull request #1888 from alphagov/cache-bust-content-api-live-tags-check-for-rummager

Bust ContentAPI cache when requesting live tags.

https://www.pivotaltracker.com/story/show/83216144