bump phpunit to ^8.0

Author: 84m

composer.json

@@ -16,7 +16,7 @@
         "elasticsearch/elasticsearch": "^7.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.0",
+        "phpunit/phpunit": "^8.0",
         "squizlabs/php_codesniffer": "^3.0"
     },
     "autoload": {