Add phpunit back.

Author: irazasyed

composer.json

@@ -26,7 +26,8 @@
     },
     "require-dev": {
         "mockery/mockery": "^1.0",
-        "orchestra/testbench": "^3.5 || ^4.0"
+        "orchestra/testbench": "^3.5 || ^4.0",
+        "phpunit/phpunit": "^6.0 || ^7.0 || ^8.0"
     },
     "autoload": {
         "psr-4": {