AFNetworking Property with 'retain (or strong)' attribute must be of object type

由于sdk低于6.0时，dispatch_queue_t  ARC没有托管，出现提示错误，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