m17n_lib: 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/28baccpjkyg7rb0alf8j2hpnczz22jzq-m17n-lib-1.8.0/bin/m17n-conv -h` got 0 exit code
- ran `/nix/store/28baccpjkyg7rb0alf8j2hpnczz22jzq-m17n-lib-1.8.0/bin/m17n-conv --help` got 0 exit code
- ran `/nix/store/28baccpjkyg7rb0alf8j2hpnczz22jzq-m17n-lib-1.8.0/bin/m17n-conv help` got 0 exit code
- ran `/nix/store/28baccpjkyg7rb0alf8j2hpnczz22jzq-m17n-lib-1.8.0/bin/m17n-conv --version` and found version 1.8.0
- ran `/nix/store/28baccpjkyg7rb0alf8j2hpnczz22jzq-m17n-lib-1.8.0/bin/m17n-date -h` got 0 exit code
- ran `/nix/store/28baccpjkyg7rb0alf8j2hpnczz22jzq-m17n-lib-1.8.0/bin/m17n-date --help` got 0 exit code
- ran `/nix/store/28baccpjkyg7rb0alf8j2hpnczz22jzq-m17n-lib-1.8.0/bin/m17n-date --version` and found version 1.8.0
- ran `/nix/store/28baccpjkyg7rb0alf8j2hpnczz22jzq-m17n-lib-1.8.0/bin/m17n-config --version` and found version 1.8.0
- found 1.8.0 with grep in /nix/store/28baccpjkyg7rb0alf8j2hpnczz22jzq-m17n-lib-1.8.0
- directory tree listing: https://gist.github.com/3d3a5a6fc658c968ce3a1298c9f32c56