ruby-cairo: update to 1.15.14.

Upstream changelog:
(from https://github.com/rcairo/rcairo/blob/v1.15.14/NEWS)

Release 1.15.14 (2018-08-20) Kouhei Sutou <kou@cozmixng.org>
============================================================

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

  * Added Cairo::Surface.create {}. Cairo::Surface.new {} is deprecated.

  * Added Cairo::Context.create {}. Cairo::Context.new {} is deprecated.

  * Windows: Added support for installing a normal gem (not fat gem)
    by "gem install --platform ruby". If you use Bundler, use "bundle
    config --local force_ruby_platform true" and "bundle exec ruby -rdevkit".

  * Changed to use SPDX license ID in gemspec.

Fixes
-----

  * Fixed a bug that CRFONTOPTIOSN2RVAL() doesn't copy the target
    options.
    * Other CR*2RVAL() copy or refer the target objects. So
      CRFONTOPTIONS2RVAL() must copy for consistency.

(tsutsui)