Let APC work under HPHP

Summary:
APC surprisingly doesn't support apc_store(array).
It supports apc_fetch(array).

Test Plan:

$ arc unit

Reviewers: epriestley, edward

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1261

Differential Revision: https://secure.phabricator.com/D4088