Use pod_target_xcconfig instead of xcconfig in the podspec

[brindy]

Hello,

This project currently causes a warning to be shown running pod install related to SWIFT_VERSION.

There is some discussion of it here:
CocoaPods/CocoaPods#5952 (comment)

The solution is apparently to use pod_target_xcconfig instead of xcconfig in the podspec.

Thanks in advance.

[iosdevzone]

This was solved in PR #63.