BREAKING - Update podspecs to support and default to the C++ bridge

Summary:
This will require people who use CocoaPods to update their Podfiles.  You can see an example of a Podfile set up to use the Cxx bridge here: https://github.com/mhorowitz/native-navigation-boilerplate/blob/master/ios/Podfile
If this doesn't work, you can continue to use the old bridge by adding a dependency of 'BatchedBridge' to the React subspecs in your Podfile, but this will stop working once the old bridge is removed.

Reviewed By: javache

Differential Revision: D4981920

fbshipit-source-id: 7c4f3bf1c3f9af3f934f03ec003a05d0cd3cb259