[Fix] ZXing broken subspec inheritance.

Triggered by fixes from #9626 and CocoaPods/CocoaPods#1523, fixed by moving to normal multi-platform support for iOS dependencies, which do inherit from the main spec. Add a dummy 'ios' subspec for backward compatibility with old Podfiles.

Unfortunately it looks like [ZXing for iOS is no longer supported](https://groups.google.com/forum/#!topic/zxing/L2oGZllqwXI), so it may be a losing battle to keep these specs functional.