Update socket.io requirement to ^2.0.4

Updates the requirements on [socket.io](https://github.com/socketio/socket.io) to permit the latest version.
- [Release notes](https://github.com/socketio/socket.io/releases)
- [Commits](https://github.com/socketio/socket.io/commits/2.0.4)

package.json

@@ -31,7 +31,7 @@
     "pump": "^1.0.2",
     "range-parser": "^1.2.0",
     "read-torrent": "^1.3.0",
-    "socket.io": "^1.7.4",
+    "socket.io": "^2.0.4",
     "torrent-stream": "^1.0.3"
   },
   "optionalDependencies": {