fix(package): update helmet to version 3.9.0

Closes #48

Author: greenkeeper[bot]

package.json

@@ -68,7 +68,7 @@
     "compression": "1.7.0",
     "dotenv": "4.0.0",
     "express": "4.15.4",
-    "helmet": "3.8.1",
+    "helmet": "3.9.0",
     "hpp": "0.2.2",
     "offline-plugin": "4.8.3",
     "prop-types": "15.5.10",