Disallow PHPUnit 8.3.0 due to https://github.com/sebastianbergmann/phpunit/issues/3772
Disallow PHPUnit 8.3.0 due to https://github.com/sebastianbergmann/phpunit/issues/3772

As discussed with @sebastianbergmann, PHPUnit 8.3.4 may fix this issue, but until then,
we lock onto an older version to have green builds to validate a release

Ocramius/ProxyManager