Use React for dependency in podspec

Author: manuquentin

RNCallKeep.podspec

@@ -13,6 +13,6 @@ Pod::Spec.new do |s|
   s.requires_arc        = true
   s.platform            = :ios, "8.0"
   s.source_files        = "ios/RNCallKeep/*.{h,m}"
-  s.dependency 'React-Core'
+  s.dependency 'React'
 end