folly: 2018.05.21.00 -> 2018.05.28.00

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 2018.05.28.00 with grep in /nix/store/j0vmqnzcn83749cw2x6rzzf3bay2zyg0-folly-2018.05.28.00
- directory tree listing: https://gist.github.com/2a05d2503ea41d5bd03239e4562bea92
- du listing: https://gist.github.com/f4cb65851fac8a956d81301fa5623140