update to latest asciidoctor-pdf

[schauder]

The latest asciidoctor-pdf version seems to be 1.5.0.alpha.13 but the example uses 1.5.0.alpha.11

I tried to simply change the version string, but the latest artefacts seem not to be available in jcenter.bintray, See https://jcenter.bintray.com/org/asciidoctor/asciidoctorj-pdf/

It would be nice to have the latest version because it probably fixes a problem I'm currently having.

[mojavelinux]

The matching AsciidoctorJ PDF hasn't been released yet. We're working on getting the repository setup so that these releases can be fully automated and thus follow on the tail of the gem release.