[Issue1] Fix undefined reference to 'Nokogiri_wrap_xml_document'

Added explicit link to avoid the undefined reference in the build on some platforms like Gentoo

To test, use branch issue1

	git clone --recursive https://github.com/krutten/nokogumbo.git
	git checkout issue1
	cd nokogumbo
	bundle install
	rake gem
	gem install pkg/nokogumbo*.gem

Tested on Gentoo with GCC 4.5.4