Update dependency gatsby to v2.15.2

Author: renovate-bot

docs/package-lock.json

@@ -5,7 +5,7 @@
     "serve": "gatsby serve"
   },
   "dependencies": {
-    "gatsby": "2.15.1",
+    "gatsby": "2.15.2",
     "gatsby-theme-apollo-docs": "2.0.2",
     "react": "16.9.0",
     "react-dom": "16.9.0"