Fix phpunit version to 6.3.*

This will fix the build error.

Author: Nyholm

composer.json

@@ -22,7 +22,7 @@
         "php-http/discovery": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.1",
+        "phpunit/phpunit": "6.3.*",
         "symfony/stopwatch": "~2.5",
         "php-http/message": "^1.0",
         "php-http/mock-client": "^1.0",