Make sure to disable OPCache for PHP 5.3

PHP 5.3:
We're using APC as opcode cache exclusively. We don't want the two PECLs
to collide.

PHP 5.4:
We're using OPCache as opcode cache. The APC is set up as user cache only
(apc.enable_opcode_cache=0), so it those two PECLs don't collide.