Add libxml-ruby as a development dependency

This is required to run the unit tests on the libxml parser.

Author: qnet-herwin

Gemfile

@@ -4,6 +4,7 @@ gemspec
 
 group :development do
   gem "bundler"
+  gem "libxml-ruby", platforms: [:ruby, :jruby]
   gem "rake"
   gem "test-unit"
 end