Update composer deps for L5.8

Author: AlexVanderbist

composer.json

@@ -22,7 +22,7 @@
         "illuminate/support": "~5.5.0|~5.6.0|~5.7.0|~5.8.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.1|^7.0",
+        "phpunit/phpunit": "^6.1|^7.0|^8.0",
         "orchestra/testbench": "~3.5.0|~3.6.0|~3.7.0|~3.8.0"
     },
     "autoload": {