chore(travis): run tests on PHP 5.3.3 and 5.6

5.3.3 is the lowest supported version, so we should test explicitly on that.

PHP 5.6 isn't stable yet, but we want to be ready for it.
Because zend framework does that. Not sure why, but if we pull it as dependency, we don't want to suffer from necessary support for 2 lowest versions of 5.3