AFNetworking 2.0 编译不过的问题修复

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