Switch to using GraphQL 15's extensions for join-monster config in a …

…schema

GraphQL 15 doesn't let schema authors attach arbitrary properties to schema objects anymore, so join-monster's config style has to change. There's an `extensions` property that works great for this, let's use that!

package.json

@@ -72,7 +72,7 @@
   },
   "homepage": "https://github.com/join-monster/join-monster#readme",
   "peerDependencies": {
-    "graphql": "0.6 || 0.7 || 0.8 || 0.9 || 0.10 || 0.11 || 0.12 || 0.13"
+    "graphql": "15"
   },
   "devDependencies": {
     "@ava/babel": "^1.0.1",
@@ -91,7 +91,7 @@
     "eslint-config-airbnb-base": "^14.1.0",
     "eslint-config-prettier": "^6.11.0",
     "faker": "^4.1.0",
-    "graphql": "^0.13.0",
+    "graphql": "^15.1.0",
     "graphsiql": "0.2.0",
     "idx": "^2.5.6",
     "jsdoc-to-markdown": "^5.0.0",