Update swift version in podspec

Kingfisher.podspec

@@ -50,5 +50,5 @@ Pod::Spec.new do |s|
   s.requires_arc = true
   s.framework = "CFNetwork"
 
-  s.pod_target_xcconfig = { 'SWIFT_VERSION' => '3.0' }
+  s.pod_target_xcconfig = { 'SWIFT_VERSION' => '4.0' }
 end