nokogiri attributes workaround support for 1.6.x 

It appears the `node['type']` field isn't being populated with later versions of nokogiri (ie 1.6.1). So the condition for finding the type based on this value fails. It can still be accessed via `node.attributes` which seems to exist in both `1.4.1` to `1.6.1`.

This should fix that AnyType VMODL error. 
https://github.com/vmware/rbvmomi/issues/31