Log Message:
graphics/ruby-chunky_png: update to 1.3.15

Update to latest 1.13 since ruby-oily_png pacakge require <1.4.

1.3.13 (2020-10-23)

* Add frozen-string-literal: true to all source files.

* Remove some backfills that were needed only for Ruby 1.8 compatibility.

1.3.14 (2020-10-27)

* Fix Canvas#inspect to not modify frozen string literal.

1.3.15 (2020-12-15)

* Set required_ruby_version to >=2.0.0 in gemspec.
* Drop dependency on SortedSet.