Update require statement for libxml_ruby 6.0.0

Requiring libxml no longer works, but `libxml-ruby` does work on older
versions too.

Author: herwinw

lib/xmlrpc/parser.rb

@@ -595,7 +595,7 @@ def parse(str)
 
     class LibXMLStreamParser < AbstractStreamParser
       def initialize
-        require 'libxml'
+        require 'libxml-ruby'
         @parser_class = LibXMLStreamListener
       end