Updated the README to require Xcode 7 beta 6.

README.md

@@ -25,7 +25,7 @@ Alamofire is an HTTP networking library written in Swift.
 ## Requirements
 
 - iOS 8.0+ / Mac OS X 10.9+ / watchOS 2
-- Xcode 7.0 beta 5+
+- Xcode 7.0 beta 6+
 
 ## Communication