valum: 0.3.14 -> 0.3.15

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. 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 0.3.15 with grep in /nix/store/7w47g1l0qg7bzc1rrmllxxinjyg7zg5c-valum-0.3.15
- found 0.3.15 in filename of file in /nix/store/7w47g1l0qg7bzc1rrmllxxinjyg7zg5c-valum-0.3.15