riot-web: 0.15.3 -> 0.15.4

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

- 0 of 0 passed binary check by having a zero exit code.
- 0 of 0 passed binary check by having the new version present in output.
- found 0.15.4 with grep in /nix/store/svsdm27wq2iipppfk9nz8k1n4q6ir9i3-riot-web-0.15.4
- directory tree listing: https://gist.github.com/62108c4884b3db3cad6d53a752a6c775
- du listing: https://gist.github.com/36270da19f3c0390457a50f0c6ecb518