Log Message:
Update ruby-libxml to 2.7.0.

== 2.7.0 / 2013-08-03 Charlie Savage

* Do not call Ruby code during a GC cycle (Dirkjan Bussink)
* Fix thread error handling (Geoffrey Giesemann)
* Set libxml error handler when a libxml-ruby error handler is set (Geoffrey \ 
Giesemann)
* Fix tests for nil TypeErrors (Geoffrey Giesemann)
* Fix segmentation fault for issue #62.  The deregisternode function was incorrectly
  NULLing out a node's mark and free functions for nodes removed from a document \ 
tree (Charlie Savage)
* Cleanup writer code (Charlie Savage)
* Fix equality check of LibXML::XML::Error against other objects (Michał Szajbe)
* Fix XML::Writer.set_quote_char UT, wrong expected string (#63).  (julp)
* Fix potential segfault when GC occurs while creating ns from xpath (Timothy \ 
Elliott)
* Fix segmentation fault after failing to load external schema (#59).  (Clifford \ 
Heath)