Log Message:
ruby-cairo: update to 1.16.6.

Upstream changes (from NEWS)

Release 1.16.6 (2020-07-23) Sutou Kouhei <kou@cozmixng.org>
============================================================

Improvements
------------

  * 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
-----

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

Thanks
------

  * Gabriel Mazetto

  * dsisnero