Since you are on a VPS, can’t you update your libmemcached version? Or use a different PHP version (7.2.) which probably comes bundled with a newer version of libmemcached?
The version never seems to change, and is 1.0.18 which is the latest as far as I can tell. In terms of PHP, the latest they offer is 7.1
How about disabling memcached in your PHP.ini?