make spot v1 compatible for PHP 5.3.3

Otherwise you get the error "Declaration of
Spot\Entity\Collection::offsetGet() must be compatible with that of
ArrayAccess::offsetGet()" . Spot v1 seems to be optimized for PHP
5.3.4+, as in there the checks for by-reference signatures were relaxed
(see http://php.net/manual/en/arrayaccess.offsetget.php)