Update ruby-nokogiri to 1.5.4.

== 1.5.4 / unreleased

* Features

  * The "nokogiri" script now has more verbose output when passed the `--rng` option. #675 (Thanks, Dan Radez!)
  * Build support on hardened Debian systems that use `-Werror=format-security`. #680.
  * Better build support for systems with pkg-config. #584
  * Better build support for systems with multiple iconv installations.

* Bugfixes

  * Segmentation fault when creating a comment node for a DocumentFragment. #677, #678.
  * Treat '.' as xpath in at() and search(). #690

  * [MRI, Security] Default parse options for XML documents were
    changed to not make network connections during document parsing,
    to avoid XXE vulnerability. #693

    To re-enable this behavior, the configuration method `nononet` may
    be called, like this:

    Nokogiri::XML::Document.parse(xml) { |config| config.nononet }

    Insert your own joke about double-negatives here.

(taca)