Set podspec version 2.1.0

Author: ishkawa

Action.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name         = "Action"
-  s.version      = "2.0.0"
+  s.version      = "2.1.0"
   s.summary      = "Abstracts actions to be performed in RxSwift."
   s.description  = <<-DESC
     Encapsulates an action to be performed, usually by a UIButton press.