add tvOS to podspec

Kingfisher.podspec

@@ -26,6 +26,7 @@ Pod::Spec.new do |s|
   s.social_media_url   = "http://twitter.com/onevcat"
 
   s.ios.deployment_target = "8.0"
+  s.tvos.deployment_target = "9.0"
   # s.osx.deployment_target = "10.7"
 
   s.source       = { :git => "https://github.com/onevcat/Kingfisher.git", :tag => s.version }