Log Message:
Update php-apcu to version 5.1.6

Changes:
- fixes gh#19: Hung apaches on pthread wrlocks
- fixes gh#188: Fix Segfault in ZTS build when locking (Tyson Andre)
- fixes gh#194: apcu_entry ttl not working
- fixes gh#189: SegFault in apc_copy_zval
- fixes gh#185: zend_mm_heap corrupted
- fixes gh#190: memory leak/failed check for duplicates