Update dependency from `React` to `React-Core`

[acrabb]

This library is now throwing an error in our iOS project:

Undefined symbols for architecture arm64:
  "_RCTRegisterModule", referenced from:
      +[RNProgressHud load] in RNProgressHud.o

Seems like a dependency needs updating as React improves
-> s.dependency "React" to s.dependency "React-Core"

See here: facebook/react-native#29633 (comment)