Provide code example that works with Xcode 6.3

Per issue #21, the Xcode 6.3 compiler will throw an "Ambiguous use of 'responseSwiftyJSON'" error using the original code sample provided. Simply adding parentheses around the callback method seems to keep the compiler happy.

Author: alexpls

README.md

@@ -11,9 +11,9 @@ Easy way to use both [Alamofire](https://github.com/Alamofire/Alamofire) and [Sw
 
 ```swift
 Alamofire.request(.GET, "http://httpbin.org/get", parameters: ["foo": "bar"])
-         .responseSwiftyJSON { (request, response, json, error) in
+         .responseSwiftyJSON({ (request, response, json, error) in
                      println(json)
                      println(error)
-                   }
+                  })
 
 ```