Add swift_version to podspec

Author: devxoul

RxKeyboard.podspec

@@ -10,6 +10,7 @@ Pod::Spec.new do |s|
   s.source_files     = 'Sources/**/*.swift'
   s.frameworks       = 'UIKit', 'Foundation'
   s.requires_arc     = true
+  s.swift_version    = "5.0"
 
   s.dependency 'RxSwift', '>= 4.4.0'
   s.dependency 'RxCocoa', '>= 4.4.0'