Updated PHPUnit dependency to 6.3

- Fixes issues when running tests against lowest supported PHPUnit
version.

Author: weierophinney

composer.json

@@ -28,7 +28,7 @@
         "psr/http-message": "^1.0.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0.8",
+        "phpunit/phpunit": "^6.3",
         "roave/security-advisories": "dev-master",
         "zendframework/zend-coding-standard": "~1.0.0"
     },