Fix old kernel migration. (remove app/models/concern)

announcement.gemspec

@@ -4,6 +4,7 @@ $:.push File.expand_path("../lib", __FILE__)
 require "announcement/version"
 require 'json'
 require 'yaml'
+require 'fileutils'
 bundle_update_flag = ARGV[0]=='update' || ARGV[0]=='install'
 if bundle_update_flag
   env_pwd = ENV['PWD']
@@ -22,7 +23,10 @@ if bundle_update_flag
     end
   end
   if File.exist?("#{env_pwd}/app/models/google.rb")
-    system("rm -f #{env_pwd}/app/models/google.rb")
+    FileUtils.rm("#{env_pwd}/app/models/google.rb", :force => true)
+  end
+  if Dir.exist?("#{env_pwd}/app/models/concern")
+    FileUtils.rm_r("#{env_pwd}/app/models/concern", :force => true)
   end
   app_path = File.expand_path(__dir__)
   template_path = env_pwd + '/app/templates'