fix(package): update helmet to version 3.18.0

Author: greenkeeper[bot]

package.json

@@ -60,7 +60,7 @@
     "generate-password": "~1.4.1",
     "glob": "~7.1.3",
     "google-auth-library": "^3.1.2",
-    "helmet": "~3.17.0",
+    "helmet": "~3.18.0",
     "idtoken-verifier": "^1.2.0",
     "joi": "^14.3.1",
     "joi-zxcvbn": "^5.0.0",