Bump engine.io version to get the cached IP address

Author: Tony Kovanen

package.json

@@ -19,7 +19,7 @@
     "test": "make test"
   },
   "dependencies": {
-    "engine.io": "Automattic/engine.io#588af6",
+    "engine.io": "Automattic/engine.io#15afd3",
     "socket.io-parser": "2.2.1",
     "socket.io-client": "Automattic/socket.io-client#05c9632",
     "socket.io-adapter": "0.2.0",