Put the swift version in the podspec so that users don't have to override it in…

Put the swift version in the podspec so that users don't have to override it in their podfile post install.

ios/permission_handler.podspec

@@ -17,5 +17,6 @@ Permission plugin for Flutter. This plugin provides a cross-platform (iOS, Andro
   s.dependency 'Flutter'
   
   s.ios.deployment_target = '8.0'
+  s.pod_target_xcconfig = { 'SWIFT_VERSION' => '4.0' }
 end