Update ruby-squib to 0.15.3.
## v0.15.3 / 2019-10-17
Chores:
* Bump pango et al to `~> 3.4`, cairo to `~ 1.16`. Fixes Ruby 2.6 Windows install issues (#287)
## v0.15.2 / 2019-09-26
Chores:
* Bump nokogiri to 1.10.4
## v0.15.1 / 2019-08-27
Chores:
* Bumped Pango et al. to 3.3.7 (#286)
## v0.15.0 / Unreleased
Features:
* Added check for malformed PNG files (#250, #218)
* Sprues: you can now flip individual cards (#251)
Bugs
* `trim_radius` now defaults to 0 on `save_pdf`, not 38. (#270)
* `explode` on `xlsx` and `csv` now defaults to `qty`, not `Qty` (#262)
Docs:
* Documented the n-sided-ness of polygons and stars
* svg: document rasterization on mask (#192)
* Document how to run the code (#186)
* Document some ways of using `prefix` in `save_png` (#268)
* `showcase` option `trim_radius` defaults to 38, not 0
* Add sample for color switch in colors docs (#274)
Chores:
* Bumped deps: Pango et al. to 3.3.6, Cairo to 1.16.4, Nokogiri to 1.10.3, Highline to 2.0.2
Compatibility:
* A LOT more pre-defined colors built into Cairo. Some of the named colors got redefined with the upgrade to the latest Cairo. This means that named colors like `:gray` and `:green` may look slightly different.
* Dropped support for EOL'd Rubies. Minimum support is 2.4
Special thanks to @lcarlsen, @Karneades
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