m17n_db: 1.7.0 -> 1.8.0

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

- built on NixOS
- ran `/nix/store/l9vsx891cc7hrp4gic87djcmbfi3dg1a-m17n-db-1.8.0/bin/m17n-db -h` got 0 exit code
- ran `/nix/store/l9vsx891cc7hrp4gic87djcmbfi3dg1a-m17n-db-1.8.0/bin/m17n-db --help` got 0 exit code
- ran `/nix/store/l9vsx891cc7hrp4gic87djcmbfi3dg1a-m17n-db-1.8.0/bin/m17n-db -v` and found version 1.8.0
- ran `/nix/store/l9vsx891cc7hrp4gic87djcmbfi3dg1a-m17n-db-1.8.0/bin/m17n-db --version` and found version 1.8.0
- ran `/nix/store/l9vsx891cc7hrp4gic87djcmbfi3dg1a-m17n-db-1.8.0/bin/m17n-db -h` and found version 1.8.0
- ran `/nix/store/l9vsx891cc7hrp4gic87djcmbfi3dg1a-m17n-db-1.8.0/bin/m17n-db --help` and found version 1.8.0
- found 1.8.0 with grep in /nix/store/l9vsx891cc7hrp4gic87djcmbfi3dg1a-m17n-db-1.8.0
- directory tree listing: https://gist.github.com/dfe2a7729da99276c71f68cf0c227fcd