use AFN 2.4 for avoid AF session progress bug

Author: longbai

Podfile

@@ -2,7 +2,7 @@ source 'https://github.com/CocoaPods/Specs.git'
 
 target "QiniuSDK iOS" do
   platform :ios, "6.0"
-  pod 'AFNetworking', '~> 2.0'
+  pod 'AFNetworking', '>= 2.4'
 end
 
 target "QiniuSDK iOSTests" do
@@ -12,7 +12,7 @@ end
 
 target "QiniuSDK Mac" do
   platform :osx, "10.8"
-  pod 'AFNetworking', '~> 2.0'
+  pod 'AFNetworking', '>= 2.4'
 end
 
 target "QiniuSDK MacTests" do