ruby-cairo: update to 1.16.6.

Upstream changes (from NEWS)

Release 1.16.6 (2020-07-23) Sutou Kouhei <kou@cozmixng.org>
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Improvements
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  * Specified GPL 2.0 or later explicitly in gemspec.
    [GitHub#62][Reported by Gabriel Mazetto]

  * Added more metadata to gem.
    [GitHub#63][Patch by Gabriel Mazetto]

Fixes
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  * Windows: Fixed a link error.
    [GitHub#ruby-gnome/ruby-gnome#1402][Reported by dsisnero]

Thanks
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  * Gabriel Mazetto

  * dsisnero

(tsutsui)