fix: docs/themes/docdock/static/revealjs/plugin/multiplex/package.json to reduce vulnerabilities (#700)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:engine.io-client:20160426
- https://snyk.io/vuln/npm:fresh:20170908
- https://snyk.io/vuln/npm:negotiator:20160616
- https://snyk.io/vuln/npm:qs:20170213
- https://snyk.io/vuln/npm:ws:20160624
- https://snyk.io/vuln/npm:ws:20171108

Author: snyk-bot

docs/themes/docdock/static/revealjs/plugin/multiplex/package.json

@@ -10,10 +10,10 @@
     "node": "~4.1.1"
   },
   "dependencies": {
-    "express": "~4.13.3",
+    "express": "~4.15.5",
     "grunt-cli": "~1.3.0",
     "mustache": "~2.2.1",
-    "socket.io": "~1.3.7"
+    "socket.io": "~1.7.4"
   },
   "license": "MIT"
 }