Add PHPUnit 8.1 compatibility

[davidkmenta]

It seems that php-mock implements some interfaces which were marked as @internal in the new version of the PHPUnit and those objects aren't backward compatible anymore 😢

[michalbundyra]

@davidkmenta Thanks for reporting the issue. I'll have a look on it later on, unless you want provide a patch? 😄

[michalbundyra]

@davidkmenta php-mock-phpunit 2.3.0 released with support for PHPUnit 8.1.