Update mumble-web to work around reconnection issue in FF

See commit ddf8424 in mumble-client

package.json

@@ -46,6 +46,6 @@
     "voice-activity-detection": "johni0702/voice-activity-detection#9f8bd90",
     "webpack": "^1.13.3",
     "mumble-client-websocket": "^1.0.0",
-    "mumble-client": "github:johni0702/mumble-client#a5cb0d7"
+    "mumble-client": "github:johni0702/mumble-client#ddf8424"
   }
 }