Only cache edition show page 404s for 5 minutes

https://www.pivotaltracker.com/story/show/46200225

Analysis of effect of changing cache times:

https://docs.google.com/a/digital.cabinet-office.gov.uk/document/d/1EDfC55ytBF7SdNgTDXoENTKBFFrgTCLTRsV8KcWcgIE/edit#heading=h.fvr0n4f7domo
Merge pull request #736 from alphagov/reduce-cache-time-for-edition-show-404s

Only cache edition show page 404s for 5 minutes

https://www.pivotaltracker.com/story/show/46200225