mbuffer: 20171011 -> 20180318

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

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

These checks were done:

- built on NixOS
- ran `/nix/store/sjlbs2k9114xlxalqcz2n0jj90lvik07-mbuffer-20180318/bin/mbuffer -h` got 0 exit code
- ran `/nix/store/sjlbs2k9114xlxalqcz2n0jj90lvik07-mbuffer-20180318/bin/mbuffer --help` got 0 exit code
- ran `/nix/store/sjlbs2k9114xlxalqcz2n0jj90lvik07-mbuffer-20180318/bin/mbuffer -V` and found version 20180318
- ran `/nix/store/sjlbs2k9114xlxalqcz2n0jj90lvik07-mbuffer-20180318/bin/mbuffer --version` and found version 20180318
- found 20180318 with grep in /nix/store/sjlbs2k9114xlxalqcz2n0jj90lvik07-mbuffer-20180318
- directory tree listing: https://gist.github.com/5b88f842903984d6134c96d89c2169de