Fix session invalid warning, only for development env

lib/rucaptcha/controller_helpers.rb

@@ -84,6 +84,8 @@ module RuCaptcha
     end
 
     def warning_when_session_invalid
+      return unless Rails.env.development?
+
       Rails.logger.warn "
         WARNING! The session.id is blank, RuCaptcha can't work properly, please keep session available.
         More details about this: https://github.com/huacnlee/rucaptcha/pull/66