[Almost there] XD-1061: Update Asciidoctor

Fixed some of the problems that appeared by just upgrading the plugin version.

The golo css is not taken into account though (thought that the plugin would try to find it, this is why I changed to doFirst, but to no avail)

see https://github.com/asciidoctor/asciidoctor-gradle-plugin/blob/master/HISTORY.adoc & https://github.com/asciidoctor/asciidoctor-gradle-plugin/issues/38