Log Message:
ruby-cairo: update to 1.16.4.

Upstream changes (from NEWS):

Release 1.16.4 (2019-03-09) Kouhei Sutou <kou@cozmixng.org>
============================================================

Fixes
-----

  * Removed Skia related code entirely.

Release 1.16.3 (2019-03-09) Kouhei Sutou <kou@cozmixng.org>
============================================================

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

  * Improved Cairo::Poinst#distance performance.
    [GitHub#53][Patch by scivola]

  * Added support for building with cairo and GLib 2.60 installed by
    Homebrew.

  * Updated colors. There are some backward incompatibility. Sorry.

Fixes
-----

  * Fixed Cairo::Color::RGB#to_s.
    [GitHub#55][Patch by spoolkitamura]

Thanks
------

  * scivola

  * spoolkitamura.