Libxml upgrade

Creating a generic interface between Tritium and Gokogiri (https://moovweb.atlassian.net/browse/PFST-49)
since the GokogiriXmlNode embeds the aml.Node type, you can directly access `InsertBefore` just by invoking it on node, i.e. `node.InsertBefore(...)`. If you think that makes the code easier to understand, change all instances. I personally don't have a strong opinion either way on this matter, but go conventions might.