Hello Jack,
the 'xml2' package is not needed. At least i don't have it installed
and i can install roo.
I think your problem is a problem with the gem 'libxml-ruby'. Do you
get the same error when you only try to install this gem 'sudo gem
install libxml-ruby'? If yes, then maybe you could ask the authors of
this gem (
http://libxml.rubyforge.org/index.xml).
-Thomas