scowl: 2017.08.24 -> 2018.04.16

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

This update was made based on information from https://repology.org/metapackage/scowl/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 2018.04.16 with grep in /nix/store/amyjmpkyvsn53x3fac7b01mwyi9jl30i-scowl-2018.04.16
- directory tree listing: https://gist.github.com/e67c69e3e02df834f3b040f36b261c13