fix(package): update prerender-node to version 3.1.1

Closes #16

Author: greenkeeper[bot]

package.json

@@ -61,7 +61,7 @@
     "compression": "^1.6.2",
     "express": "^4.15.2",
     "prerender": "^4.4.1",
-    "prerender-node": "^2.7.1",
+    "prerender-node": "^3.1.1",
     "update-notifier": "^2.2.0",
     "witch": "latest"
   }