memcached: 1.5.4 -> 1.5.5

Semi-automatic update. These checks were performed:

- built on NixOS
- ran `/nix/store/6b8zx40j50ccjwn2z2s94zsgbzyghzxy-memcached-1.5.5/bin/memcached -h` got 0 exit code
- ran `/nix/store/6b8zx40j50ccjwn2z2s94zsgbzyghzxy-memcached-1.5.5/bin/memcached --help` got 0 exit code
- ran `/nix/store/6b8zx40j50ccjwn2z2s94zsgbzyghzxy-memcached-1.5.5/bin/memcached -V` and found version 1.5.5
- ran `/nix/store/6b8zx40j50ccjwn2z2s94zsgbzyghzxy-memcached-1.5.5/bin/memcached --version` and found version 1.5.5
- ran `/nix/store/6b8zx40j50ccjwn2z2s94zsgbzyghzxy-memcached-1.5.5/bin/memcached -h` and found version 1.5.5
- ran `/nix/store/6b8zx40j50ccjwn2z2s94zsgbzyghzxy-memcached-1.5.5/bin/memcached --help` and found version 1.5.5
- found 1.5.5 with grep in /nix/store/6b8zx40j50ccjwn2z2s94zsgbzyghzxy-memcached-1.5.5
- found 1.5.5 in filename of file in /nix/store/6b8zx40j50ccjwn2z2s94zsgbzyghzxy-memcached-1.5.5

cc "@coconnor"