Update to phpunit 8
This branch contains updates the phpunit dependency in zon-core from ~4.8.0 to ~8.0.
It was part of the phpunit 8 upgrade I did for AFES a little while back, in MR 267 in the AFES project; I should have issued this MR at the time.
Will need to consider if other projects are currently using the zon-core feature/encryption branch though, maybe?