PropertyAccess::createPropertyAccessor() doesn't work with Symfony 2.2

`PropertyAccess::createPropertyAccessor()` only works with Symfony 2.3, I'd suggest to use `PropertyAccess::getPropertyAccessor()` (which works in both s2.2 and s2.3) until Symfony 3.0 becomes available.

http://symfony.com/doc/current/components/property_access/introduction.html#usage
https://github.com/symfony/PropertyAccess/blob/master/PropertyAccess.php#L46