AFNetworking 2.0 当Deployment Target 低于6.0时，AFURLConnectionOperation.h, AFURLSessionManager.h
@property (nonatomic, strong) dispatch_queue_t completionQueue;
由于sdk低于6.0时，dispatch_queue_t ARC没有托管，出现提示错误
Property with 'retain (or strong)' attribute must be of object type
修改为
#if OS_OBJECT_USE_OBJC
@property (nonatomic, strong) dispatch_queue_t completionQueue;
#else
@property (nonatomic, assign) dispatch_queue_t completionQueue;
#endif
#if OS_OBJECT_USE_OBJC
@property (nonatomic, strong) dispatch_group_t completionGroup;
#else
@property (nonatomic, assign) dispatch_group_t completionGroup;
#endif
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