Fix phpunit version to 6.3.*

Nyholm · Nyholm · commit 711264afa5df · 2017-10-06T09:25:36.000-04:00
This will fix the build error.
diff --git a/composer.json b/composer.json
@@ -19,7 +19,7 @@
         "php-http/promise": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.1",
+        "phpunit/phpunit": "6.3.*",
         "cache/void-adapter": "^1.0"
     },
     "autoload": {
