chore(deps): update dependency @types/node-fetch to v2.5.6

Author: renovate-bot

package.json

@@ -87,7 +87,7 @@
     "@types/mocha": "5.2.7",
     "@types/mongodb": "3.1.10",
     "@types/node": "9.6.55",
-    "@types/node-fetch": "2.5.3",
+    "@types/node-fetch": "2.5.6",
     "@types/object-hash": "1.3.1",
     "@types/redis": "2.8.6",
     "@types/source-map-support": "0.5.1",