Fix Xcode 12 compatibility. (#1954)

Author: jinshin1013

CodePush.podspec

@@ -20,7 +20,7 @@ Pod::Spec.new do |s|
   # Note: Even though there are copy/pasted versions of some of these dependencies in the repo, 
   # we explicitly let CocoaPods pull in the versions below so all dependencies are resolved and 
   # linked properly at a parent workspace level.
-  s.dependency 'React'
+  s.dependency 'React-Core'
   s.dependency 'SSZipArchive', '~> 2.2.2'
   s.dependency 'JWT', '~> 3.0.0-beta.12'
   s.dependency 'Base64', '~> 1.1'