Update Helmet version to 6.1.5

Author: EvanHahn

_config.yml

@@ -1,7 +1,7 @@
 title: Helmet
 short_description: Express.js security with HTTP headers
 description: Helmet helps you secure your Express.js apps by setting various HTTP headers. It's not a silver bullet, but it can help!
-helmet_version: "6.1.4"
+helmet_version: "6.1.5"
 baseurl: ""
 url: "https://helmetjs.github.io"