Download this package and expand it:

ftp://xmlsoft.org/libxml2/libxml2-2.7.3.tar.gz

Expand the archive and cd into its directory

./configure && make && sudo make install

At this point, you need to recompile nokogiri with the new libxml:

sudo gem install nokogiri——with-xml2-include=/usr/local/include/libxml2—with-xml2-lib=/usr/local/lib—with-xslt-dir=/usr/local