fix(package): update helmet to version 3.18.0

Closes #67

Author: greenkeeper[bot]

package.json

@@ -64,7 +64,7 @@
     "flat": "4.1.0",
     "fs-extra": "7.0.1",
     "github-api": "3.0.0",
-    "helmet": "3.15.0",
+    "helmet": "3.18.0",
     "js-yaml": "3.12.1",
     "octonode": "0.9.5",
     "private-ip": "1.0.5",