Log Message:
Update ruby-cairo to 1.15.9.

Release 1.15.9 (2017-06-03) Kouhei Sutou <kou@cozmixng.org>
===========================================================

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

  * Supported RubyInstaller2.
    [GitHub#47][Reported by PLURIX]

Thanks
------

  * PLURIX

Release 1.15.8 (2017-05-29) Kouhei Sutou <kou@cozmixng.org>
===========================================================

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

  * Added Cairo::FreeTypeFotnFace.
    [GitHub#45][Patch by Stafford Brunk]

  * Windows: Supported Ruby 2.4.
    [ruby-list:50525][Reported by 5.5]

  * Windows: Supported MSYS2.

  * Windows: Updated bundled libraries:
    * zlib: 1.2.8 -> 1.2.11

Thanks
------

  * Stafford Brunk

  * 5.5

Release 1.15.7 (2017-05-01) Kouhei Sutou <kou@cozmixng.org>
===========================================================

Fixes
-----

  * Windows: Fixed zlib version. We should use the same version
    as Ruby Installer. Ruby Installer uses zlib 1.2.8.
    [GitHub#47][Reported by jjgh]

Thanks
------

  * jigh

Release 1.15.6 (2017-04-27) Kouhei Sutou <kou@cozmixng.org>
===========================================================

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

  * Added Cairo::Context#refrence_count.
  * Windows: Updated bundled libraries:
    * zlib: 1.2.10 -> 1.2.11
    * libpng: 1.6.27 -> 1.6.29
    * FreeType: 2.7 -> 2.7.1
  * Used native-package-installer gem.