Add React to podspec

Inspired by charlires#33

Apparently this worked fine with Cocoapods 1.4.0, but with Cocoapods 1.5.0 we need to specify the dependency on React.
ocetnik/react-native-background-timer#82 (comment)

Author: idris

ios/SegmentAnalytics.podspec

@@ -29,6 +29,7 @@ Pod::Spec.new do |s|
 
   s.source_files = 'SegmentAnalytics/Classes/**/*.{h,m}'
 
+  s.dependency 'React'
   s.dependency 'Analytics'
   s.dependency 'segment-appsflyer-ios'
 end