Add main in conference sample package.json (#339)

Author: starwarfan

src/samples/conference/package.json

@@ -20,6 +20,7 @@
     "socket.io": "^2.2.0",
     "spdy": ""
   },
+  "main": "samplertcservice.js",
   "engines": {
     "node": ">=0.10"
   }