Fix Travis CI

Author: sangnguyenplus

composer.json

@@ -20,7 +20,7 @@
         "laravel/framework": "^5.6|^6.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.1|^7.0|^8.0",
+        "phpunit/phpunit": "^7.0|^8.0",
         "mockery/mockery": "^1.0"
     },
     "autoload": {