Set created_at as collection cache key

[bmedenwald]

With Rails Conditional GET support (304 caching), the stale? and fresh_when methods rely on collection_cache_key, which by default utilize the updated_at column. Because the audits table doesn't include an updated_at column, this throws an SQL error.

This change simply changed the default and relies on super.

[jaredbeck]

Would a better title for this PR be "SQL error in Rails Conditional GET (304 caching)"? Might help searchers in the future.