Fixes an issue when using AFURLSessionManager on unsupported OS 

0a3de62849224ad0f878c801c4aa0a33edc30a98 allowed building this for iOS 6 and OS X 10.8 however it didn't prevent running it. When people run it, NSURLSession will become nil and it can cause some issues when using AFURLSessionManager such as in #1592. Let's just bail early and prevent the developer for continuing.