chore(package): update graphql to version 15.0.0

Author: greenkeeper[bot]

package.json

@@ -54,7 +54,7 @@
     "@types/simple-mock": "0.8.0",
     "chai": "^4.2.0",
     "chai-as-promised": "^7.1.1",
-    "graphql": "^14.6.0",
+    "graphql": "^15.0.0",
     "graphql-subscriptions": "^1.0.0",
     "ioredis": "^4.16.0",
     "istanbul": "1.0.0-alpha.2",