Update HTTP server example for reactphp/http v1.6.0

Author: clue

README.md

@@ -24,11 +24,7 @@ components to create async network clients/servers and more.
 
 ```php
 $server = new React\Http\HttpServer(function (Psr\Http\Message\ServerRequestInterface $request) {
-    return new React\Http\Message\Response(
-        200,
-        array(
-            'Content-Type' => 'text/plain'
-        ),
+    return React\Http\Message\Response::plaintext(
         "Hello World!\n"
     );
 });

composer.json

@@ -12,7 +12,7 @@
         "react/cache": "^1.0",
         "react/dns": "^1.8",
         "react/event-loop": "^1.2",
-        "react/http": "^1.5",
+        "react/http": "^1.6",
         "react/promise": "^2.1 || ^1.2",
         "react/promise-stream": "^1.1.1",
         "react/promise-timer": "^1.7",