When "apcu_clear_cache()" exists, prefer it as a cache clear callback over "apc_clear_cache()"

Summary:
See PHI36. APCu originally had apc_ methods, but at some point dropped these and only provides apcu_ methods.

When the apcu_ method is present, use it. It may not be present for older versions of APCu, so keep the fallback.

Test Plan:

• With modern APCu, clicked "Purge Caches" in Config > Caches.
• Before: fatal on bad apc_clear_caches call.
• After: Valid cache clear.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D18449