Log Message:
Update ruby-libxml to 2.8.0.

= Release History

== 2.8.0 / 2015-01-09 Charlie Savage

* Use RbConfig instead of Config in extconf.rb (Robert Haines)
* Correct xpath documentation XML (Bill Mill)
* Correct from_string method documentation (Bill Mill)
* Fix compile error with debug not enabled in libxml (Patrick Ziegler)
* Update gemspec to include license (Charlie Savage)
* In XML::Writer prevent writing to io while being GCed (Brett Gibson)