Update composer.json

Author: barryvdh

composer.json

@@ -40,7 +40,7 @@
     "require-dev": {
         "squizlabs/php_codesniffer": "^2.3",
         "orchestra/testbench": "3.x",
-        "phpunit/phpunit": "^4.8|^5.2"
+        "phpunit/phpunit": "^4.8|^5.2|^6"
     },
     "scripts": {
         "test": "phpunit",