For any who are still getting "libxml2 version 2.6.21 or later is required!" message, removing --use-system-libraries from hlv's step 2 above, worked for me.
sudo env ARCHFLAGS="-arch x86_64" gem install nokogiri:1.6.4.1 -- --with-xml=/usr/local/Cellar/libxml2/2.9.2/