memcached: 1.5.5 -> 1.5.6

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- ran `/nix/store/d6hf5flqvv4cw0pfbbnx8m99gs1apsk0-memcached-1.5.6/bin/memcached -h` got 0 exit code
- ran `/nix/store/d6hf5flqvv4cw0pfbbnx8m99gs1apsk0-memcached-1.5.6/bin/memcached --help` got 0 exit code
- ran `/nix/store/d6hf5flqvv4cw0pfbbnx8m99gs1apsk0-memcached-1.5.6/bin/memcached -V` and found version 1.5.6
- ran `/nix/store/d6hf5flqvv4cw0pfbbnx8m99gs1apsk0-memcached-1.5.6/bin/memcached --version` and found version 1.5.6
- ran `/nix/store/d6hf5flqvv4cw0pfbbnx8m99gs1apsk0-memcached-1.5.6/bin/memcached -h` and found version 1.5.6
- ran `/nix/store/d6hf5flqvv4cw0pfbbnx8m99gs1apsk0-memcached-1.5.6/bin/memcached --help` and found version 1.5.6
- found 1.5.6 with grep in /nix/store/d6hf5flqvv4cw0pfbbnx8m99gs1apsk0-memcached-1.5.6
- directory tree listing: https://gist.github.com/4f05e6b4a66fc90f9ebcb02b05aead27