minor #2533 AppVeyor - adjust phpunit version (keradus)
minor #2533 AppVeyor - adjust phpunit version (keradus)

This PR was merged into the 2.1 branch.

Discussion
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AppVeyor - adjust phpunit version

adjusting PHPUnit version used by AppVeyor after unfreezing AppVeyor.
ref https://github.com/appveyor/ci/issues/1312

Commits
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e28e05f1 AppVeyor - adjust phpunit version

kubawerlos/PHP-CS-Fixer,FriendsOfPHP/PHP-CS-Fixer,SpacePossum/PHP-CS-Fixer,felixgomez/PHP-CS-Fixer,keradus/PHP-CS-Fixer,keradus/PHP-CS-Fixer,kubawerlos/PHP-CS-Fixer,Soullivaneuh/PHP-CS-Fixer,gharlan/PHP-CS-Fixer,lucasmichot/PHP-CS-Fixer,SpacePossum/PHP-CS-Fixer,Soullivaneuh/PHP-CS-Fixer,FriendsOfPHP/PHP-CS-Fixer,gharlan/PHP-CS-Fixer,GrahamCampbell/PHP-CS-Fixer,Slamdunk/PHP-CS-Fixer,lucasmichot/PHP-CS-Fixer,felixgomez/PHP-CS-Fixer,Slamdunk/PHP-CS-Fixer,GrahamCampbell/PHP-CS-Fixer