Issue #19: fix mem_requested key change in Memcached 1.5.17
Issue #19: fix mem_requested key change in Memcached 1.5.17

- reference: https://github.com/memcached/memcached/wiki/ReleaseNotes1517
- fixe doesn't use a null coalescing to avoid requireing PHP 7.

wp-cloud/phpmemcacheadmin,wp-cloud/phpmemcacheadmin,wp-cloud/phpmemcacheadmin