folly: 2018.04.23.00 -> 2018.05.21.00

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

This update was made based on information from https://repology.org/metapackage/folly/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 2018.05.21.00 with grep in /nix/store/n4qdpjqixmlcly3al00c5a2ivp6lh74f-folly-2018.05.21.00
- directory tree listing: https://gist.github.com/80adf61ca8bb6b1020e7b48cae590a9c
- du listing: https://gist.github.com/dc5d5cb97ea76a087a50b733c659ebe5