Change "Iterable" interface to "Traversable"

The interface originally implemented in the UString class documented in the `README.md` file was an invalid "Iterable" interface.

PHP has no "Iterable" interface, however a ["Traversable"][php-traversable] interface does exist (and seems to have been the intended choice.

[php-traversable]: http://php.net/manual/en/class.traversable.php