Fix phpunit version to 6.3.*

This will fix the build error.

Author: Nyholm

composer.json

@@ -15,7 +15,7 @@
         "php": "^7.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.1",
+        "phpunit/phpunit": "6.3.*",
         "symfony/stopwatch": "~2.5",
         "nyholm/nsa": "^1.1"
     },