Remove Swift 4 support from podspec

Kingfisher.podspec

@@ -25,8 +25,7 @@ Pod::Spec.new do |s|
   s.authors            = { "onevcat" => "onevcat@gmail.com" }
   s.social_media_url   = "https://github.com/onevcat"
 
-  s.swift_version = "5.0"
-  s.swift_versions = ['4.0', '4.2', '5.0']
+  s.swift_versions = ['5.0']
 
   s.ios.deployment_target = "10.0"
   s.tvos.deployment_target = "10.0"