[#73135884] Increase TTL expiry time to 24 hours

Increase the TTL expiry time because the full crawl is currently
taking just over 12 hours. As a result, we end up going round a number
of URLs again after 12 hours have passed.
Merge pull request #29 from alphagov/increase-ttl-expiry-to-24-hours

[#73135884] Increase TTL expiry time to 24 hours