use relativize path from phpcr-utils

fix https://github.com/jackalope/jackalope-doctrine-dbal/commit/c203779fc8387150bc1fdfee3094705339f04317#commitcomment-7436081

needs https://github.com/phpcr/phpcr-utils/pull/123 to be merged first
i think phpcr-utils 1.x may not do BC breaks anymore, so this should be save.