Update ruby-nokogiri to 1.5.6.

== 1.5.6 / unreleased

* Features

  * Improved performance of XML::Document#collect_namespaces. #761 (Thanks, Juergen Mangler!)
  * New callback SAX::Document#processing_instruction (Thanks, Kitaiti Makoto!)
  * Node#native_content= allows setting unescaped node contant. #768
  * XPath lookup with namespaces supports symbol keys. #729 (Thanks, Ben Langfeld.)
  * XML::Node#[]= stringifies values. #729 (Thanks, Ben Langfeld.)
  * bin/nokogiri will process a document from $stdin
  * bin/nokogiri -e will execute a program from the command line
  * bin/nokogiri --version will print the Xerces and NekoHTML versions when ran with JRuby.

* Bugfixes
  * Nokogiri now detects XSLT transform errors. #731 (Thanks, Justin Fitzsimmons!)
  * Don't throw an Error when trying to replace top-level text node in DocumentFragment. #775
  * Raise an ArgumentError if an invalid encoding is passed to the SAX parser. #756 (Thanks, Bradley Schaefer!)
  * [JRuby] space prior to xml preamble causes nokogiri to fail parsing. (fixed along with #748) #790
  * [JRuby] Fixed the bug Nokogiri::XML::Node#content inconsistency between Java and C. #794, #797
  * [JRuby] raises INVALID_CHARACTER_ERR exception when EntityReference name starts with '#'. #719
  * [JRuby] doesn't coerce namespaces out of strings on a direct subclass of Node. #715
  * [JRuby] Node#content now renders newlines properly. #737 (Thanks, Piotr Szmielew!)
  * [JRuby] Unknown namespace are ignore when the recover option is used. #748
  * [JRuby] XPath queries for namespaces should not throw exceptions when called twice in a row. #764
  * [JRuby] More consistent (with libxml2) whitespace formatting when emitting XML. #771
  * [JRuby] namespaced attributes broken when appending raw xml to builder. #770
  * [JRuby] Nokogiri::XML::Document#wrap raises undefined method `length' for nil:NilClass when trying to << to a node. #781
  * [JRuby] Fixed "bad file descriptor" bug when closing open file descriptors. #495
  * [JRuby] JRuby/CRuby incompatibility for attribute decorators. #785
  * [JRuby] Issues parsing valid XML with no internal subset in the DTD. #547, #811
  * [JRuby] Issues parsing valid node content when it contains colons. #728
  * [JRuby] Correctly parse the doc type of html documents. #733
  * [JRuby] Include dtd in the xml output when a builder is used with create_internal_subset. #751
  * [JRuby] builder requires textwrappers for valid utf8 in jruby, not in mri. #784