magento/graphql-ce#777: Upgrade graphql-php library to the latest version

Author: lenaorobei

composer.json

@@ -52,7 +52,7 @@
         "symfony/process": "~4.1.0|~4.2.0|~4.3.0",
         "tedivm/jshrink": "~1.3.0",
         "tubalmartin/cssmin": "4.1.1",
-        "webonyx/graphql-php": "^0.13.6",
+        "webonyx/graphql-php": "^0.13.8",
         "zendframework/zend-captcha": "^2.7.1",
         "zendframework/zend-code": "~3.3.0",
         "zendframework/zend-config": "^2.6.0",