Update podspec

Author: KyoheiG3

SpringIndicator.podspec

@@ -1,10 +1,11 @@
 Pod::Spec.new do |s|
   s.name         = "SpringIndicator"
-  s.version      = "3.0.0"
+  s.version      = "4.0.0"
   s.summary      = "SpringIndicator is a indicator such as a spring and PullToRefresh."
   s.homepage     = "https://github.com/KyoheiG3/SpringIndicator"
   s.license      = { :type => "MIT", :file => "LICENSE" }
   s.author       = { "Kyohei Ito" => "[email protected]" }
+  s.swift_version = '4.2'
   s.ios.deployment_target = '8.0'
   s.tvos.deployment_target = '9.0'
   s.source       = { :git => "https://github.com/KyoheiG3/SpringIndicator.git", :tag => s.version.to_s }