Bump minimum required version of PHP to 5.3.9.

This change is needed to address a bug in older versions of PHP 5.3
affecting inheritance (see https://bugs.php.net/bug.php?id=66818).

We will most likely resort to a workaround in v1.0 to stick with the
currently required minimum version of PHP (>= 5.3.2), but v1.1 will
surely require => 5.3.9.

[ci skip]