linuxPackages.wireguard: 0.0.20180413 -> 0.0.20180420

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

This update was made based on information from https://repology.org/metapackage/wireguard/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.)
- directory tree listing: https://gist.github.com/62dbb660d0b10010e5638c9be376d68b