riot-web: 0.13.5 -> 0.14.0

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

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

These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 0.14.0 with grep in /nix/store/rrr1riavdg8sysb7ksvfqwr9az3jvlbm-riot-web-0.14.0
- directory tree listing: https://gist.github.com/586766758d4a74da4f7c241391412e10