Upgrade to phpunit 8

Author: fabacino

composer.json

@@ -14,7 +14,7 @@
         "psr/log": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0"
+        "phpunit/phpunit": "^8.0"
     },
     "autoload": {
         "psr-4": {