Add swift_version to spec

* Specifies the swift version that will be used for the pod regardless of the swift version the app target uses.

Note: This feature was added on CocoaPods 1.4.0 (http://blog.cocoapods.org/CocoaPods-1.4.0/)

Kingfisher.podspec

@@ -25,6 +25,8 @@ Pod::Spec.new do |s|
   s.authors            = { "onevcat" => "onevcat@gmail.com" }
   s.social_media_url   = "http://twitter.com/onevcat"
 
+  s.swift_version = "4.0"
+
   s.ios.deployment_target = "8.0"
   s.tvos.deployment_target = "9.0"
   s.osx.deployment_target = "10.10"