chore(package): update helmet to version 3.20.0

Author: greenkeeper[bot]

package.json

@@ -3,7 +3,7 @@
     "axios": "^0.19.0",
     "compression": "^1.7.1",
     "express": "^4.16.2",
-    "helmet": "^3.12.1",
+    "helmet": "^3.20.0",
     "humps": "^2.0.1",
     "immutable": "^3.8.2",
     "next": "^9.0.0",