fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:ms:20170412

Latest report for w33ble/customer-manager-vue-demo:
https://snyk.io/test/github/w33ble/customer-manager-vue-demo

package.json

@@ -13,7 +13,7 @@
   "dependencies": {
     "compression": "^1.0.3",
     "cors": "^2.5.2",
-    "helmet": "^1.3.0",
+    "helmet": "^3.6.1",
     "isarray": "~1.0.0",
     "loopback": "^3.0.0",
     "loopback-boot": "^2.6.5",