Update bundled gems list

Author: nagachika

gems/bundled_gems

@@ -10,7 +10,7 @@ minitest            5.25.1  https://github.com/minitest/minitest
 power_assert        2.0.4   https://github.com/ruby/power_assert
 rake                13.2.1  https://github.com/ruby/rake
 test-unit           3.6.2   https://github.com/test-unit/test-unit
-rexml               3.3.8   https://github.com/ruby/rexml
+rexml               3.3.9   https://github.com/ruby/rexml
 rss                 0.3.1   https://github.com/ruby/rss
 net-ftp             0.3.8   https://github.com/ruby/net-ftp
 net-imap            0.4.17  https://github.com/ruby/net-imap