Add monkey patch to avoid rexml parsing error

REXML fails to parse some xml response from xmpp server and get the following error.

  REXML::ParseException
  #<Encoding::CompatibilityError: incompatible encoding regexp match (UTF-8 regexp with ASCII-8BIT string)>
  .rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/rexml/source.rb:212:in `match'

This causes bot to fail to join some rooms.

See https://github.com/lnussbaum/xmpp4r/issues/3