Bump up the default cache size.
Bump up the default cache size.

The original cache size of 10 items dates from the distant past (commits
5906c7b825b24bf38df3 and 938515daae188ee6b1b) before we had any kind of
cache expiry, so the low limit served to prevent stale items hanging
around for too long. We don't need that any more.

alphagov/gds-api-adapters,bitzesty/gds-api-adapters,bitzesty/gds-api-adapters,theodi/gds-api-adapters,theodi/gds-api-adapters