Set minimum RxPHP version to 2.0.4

Author: davidwdan

composer.json

@@ -17,7 +17,7 @@
     "ocramius/package-versions": "^1.1",
     "react/dns": "^0.4.9",
     "react/http-client": "^0.4.17",
-    "reactivex/rxphp": "^2.0",
+    "reactivex/rxphp": "^2.0.4",
     "rx/websocket": "^1.0",
     "voryx/event-loop": "^2.0.2"
   },