Update dependency mustache to v4

Author: renovate-bot

reveal.js/plugin/multiplex/package.json

@@ -12,7 +12,7 @@
   "dependencies": {
     "express": "4.13.4",
     "grunt-cli": "0.1.13",
-    "mustache": "2.2.1",
+    "mustache": "4.0.1",
     "socket.io": "1.3.7"
   },
   "license": "MIT"