abcmidi: 2018.03.08 -> 2018.03.21

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

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

These checks were done:

- built on NixOS
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/abc2midi -h` got 0 exit code
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/midi2abc -h` got 0 exit code
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/midi2abc --help` got 0 exit code
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/abc2abc -h` got 0 exit code
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/mftext -h` got 0 exit code
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/mftext --help` got 0 exit code
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/mftext help` got 0 exit code
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/mftext -V` and found version 2018.03.21
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/mftext -v` and found version 2018.03.21
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/mftext --version` and found version 2018.03.21
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/mftext -h` and found version 2018.03.21
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/mftext --help` and found version 2018.03.21
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/yaps -h` got 0 exit code
- ran `/nix/store/21xl9z5m4rag6rhc5jhy3ik15vdzj8dd-abcMIDI-2018.03.21/bin/abcmatch -h` got 0 exit code
- directory tree listing: https://gist.github.com/4a5c8556345e7bb0a99964ec83e3384b