React Native 0.40 support

According to https://github.com/facebook/react-native/releases/tag/v0.40.0,

Native code on iOS must refer to headers out of the react namespace. Previously the following would work:

#import "RCTUtils.h"

But now all headers have been moved:

#import <React/RCTUtils.h>

As far as it's a breaking change, I bumped the version to 2.0.

Peace ✌️