Fix Xcode 12 Compatibility (#672)

Xcode 12 (released on Sept 16) fails to build without the module being liked to React-Core directly. This change is needed for React Native 0.60.2 or newer for iOS
Ref: facebook/react-native#29633 (comment)

Co-authored-by: Yogev Ben David <[email protected]>

Author: jaykhandelwal

react-native-notifications.podspec

@@ -20,5 +20,5 @@ Pod::Spec.new do |s|
   s.source_files   = 'lib/ios/*.{h,m}'
   s.exclude_files  = "lib/ios/RNNotificationsTests/**/*.*", "lib/ios/OCMock/**/*.*"
 
-  s.dependency 'React'
+  s.dependency 'React-Core'
 end