Encourage to use PHPUnit 8

composer.json

@@ -19,7 +19,7 @@
         "fzaninotto/faker": "^1.4",
         "mockery/mockery": "^1.0",
         "nunomaduro/collision": "^2.0",
-        "phpunit/phpunit": "^7.5"
+        "phpunit/phpunit": "^8.0"
     },
     "config": {
         "optimize-autoloader": true,