chore(package): update helmet to version 3.1.0

https://greenkeeper.io/

package.json

@@ -164,7 +164,7 @@
     "dompurify": "0.8.4",
     "express": "4.14.0",
     "extract-text-webpack-plugin": "1.0.1",
-    "helmet": "2.3.0",
+    "helmet": "3.1.0",
     "isomorphic-fetch": "2.2.1",
     "jed": "1.1.1",
     "jsdom": "9.8.3",