Support PHPUnit 8

Author: Naktibalda

composer.json

@@ -12,7 +12,7 @@
     "require": {
         "php": ">=7.1.0",
         "myclabs/deep-copy": "~1.1",
-        "phpunit/phpunit": "^7.0"
+        "phpunit/phpunit": ">=7.0 <9.0"
     },
     "autoload": {
         "psr-0": {