Set organisations show cache max age to 5mins

Lazy load an @cache_max_age variable and override it on organisations
show

Update Organisations controller test to use 5mins cache max age
Set the cache to 5 mins when no publications are scheduled

expires_in 5 mins which will be overdidden by
expire_on_next_scheduled_publication closer to publishing time.
Merge pull request #117 from alphagov/orgs-show-cache-5-mins

Set organisations show cache max age to 5mins