updated testing package requirements

Author: tylercd100

composer.json

@@ -32,7 +32,7 @@
         "php": "^7.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0",
+        "phpunit/phpunit": "^6.3",
         "orchestra/testbench": "^3.5"
     },
     "suggest": {