Update ruby-rcairo to 1.12.6.

Release 1.12.6 (2013-05-24) Kouhei Sutou <kou@cozmixng.org>
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Improvements
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  * Supported Windows XP again.
    [ruby-list:49392][Reported by 5.5]
    [ruby-list:49401][Researched by myokoym]

Thanks
------

  * 5.5
  * myokoym

Release 1.12.5 (2013-05-18) Kouhei Sutou <kou@cozmixng.org>
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Improvements
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  * Updated bundled cairo binary for Windows to 1.12.14.

(obache)