Add in the Memcached extension class constants

RES_INVALID_HOST_PROTOCOL and RES_NOT_SUPPORTED

These are added in Zends memcached extension, see [here](https://github.com/php-memcached-dev/php-memcached/blob/master/php_memcached.c#L4382) and [here](https://github.com/php-memcached-dev/php-memcached/blob/master/php_memcached.c#L4386). I verified that the constants are now accessible as normal Memcached constants; this struck me as a bit too trivial for a test case, let me know if one is needed.