Prevent pulling in 2.0 dependency

Projects supporting anything lower than iOS 7 that also use AFDownloadRequestOperation will not be able to `pod install` because the dependency in the podspec jumps up to AFNetworking 2.0 beta (arguably a bug in CocoaPods, see [#1263](https://github.com/CocoaPods/CocoaPods/issues/1263)). 

This change allows the dependent version of AFNetworking to increment but keeps it below 2.0.