fix(podspec): Lowered deployment target for MacOS to 10.13 (#1673)

react-native-webview.podspec

 
   s.authors      = package['author']
   s.homepage     = package['homepage']
-  s.platforms    = { :ios => "9.0", :osx => "10.14" }
+  s.platforms    = { :ios => "9.0", :osx => "10.13" }
 
   s.source       = { :git => "https://github.com/react-native-community/react-native-webview.git", :tag => "v#{s.version}" }
   s.source_files  = "apple/**/*.{h,m}"