Update dependency helmet to v3.15.0 (#850)

Author: renovate[bot]

appengine/loopback/package.json

@@ -18,7 +18,7 @@
   "dependencies": {
     "compression": "1.7.2",
     "cors": "2.8.4",
-    "helmet": "3.12.0",
+    "helmet": "3.15.0",
     "loopback-boot": "2.27.1",
     "loopback-component-explorer": "5.3.0",
     "serve-favicon": "2.4.5",