This issue was solved with changeset r42474.
Steve, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
lib/rss/rexmlparser.rb: Remove needless REXML version check.
Both RSS Parser and REXML are bundled in Ruby. RSS Parser can
always use the latest REXML. [Bug #8754] [ruby-core:56454]
Patch by Steve Klabnik. Thanks!!!