memcached: 1.5.6 -> 1.5.7

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/memcached/versions.

These checks were done:

- built on NixOS
- ran `/nix/store/mkaiblx8rmxg196cx0zav38kmask6vjy-memcached-1.5.7/bin/memcached -h` got 0 exit code
- ran `/nix/store/mkaiblx8rmxg196cx0zav38kmask6vjy-memcached-1.5.7/bin/memcached --help` got 0 exit code
- ran `/nix/store/mkaiblx8rmxg196cx0zav38kmask6vjy-memcached-1.5.7/bin/memcached -V` and found version 1.5.7
- ran `/nix/store/mkaiblx8rmxg196cx0zav38kmask6vjy-memcached-1.5.7/bin/memcached --version` and found version 1.5.7
- ran `/nix/store/mkaiblx8rmxg196cx0zav38kmask6vjy-memcached-1.5.7/bin/memcached -h` and found version 1.5.7
- ran `/nix/store/mkaiblx8rmxg196cx0zav38kmask6vjy-memcached-1.5.7/bin/memcached --help` and found version 1.5.7
- found 1.5.7 with grep in /nix/store/mkaiblx8rmxg196cx0zav38kmask6vjy-memcached-1.5.7
- directory tree listing: https://gist.github.com/affc1a31a9e5b9194408621fe24a9f04