CocoaPods升级AFNetworking-4.0报错！

CocoaPods could not find compatible versions for pod "AFNetworking":
In Podfile:
AFNetworking (~> 4.0.1)

Specs satisfying the AFNetworking (~> 4.0.1) dependency were found, but they required a higher minimum deployment target.

今天升级AFNetworking报错，原因是podfile文件里的platform :ios, '8.0'。
AFNetworking需要9.0以上。