Set the versions in package.json and React.podspec to 0.0.0-master

Summary:
The current versions in these files is 0.12.0, which is out of date. Better to claim no version than the wrong version, so this diff changes the versions to 0.0.0-master.

Release branches will still have the correct versions.
Closes https://github.com/facebook/react-native/pull/5241

Reviewed By: svcscm

Differential Revision: D2819277

Pulled By: javache

fb-gh-sync-id: 99d4682e5d1f571e241f306caf309b2944483909