Update asciidoctor-plantuml gem to v0.0.12

This adds support for the deflate encoding: https://github.com/hsanson/asciidoctor-plantuml/pull/12.

Currently this can only be changed by setting the PLANTUML_ENCODING environment variable to deflate:

gitlab_rails['env'] = { 'PLANTUML_ENCODING' => 'deflate' }

Relates to #216587 (closed)

Test notes

See https://docs.gitlab.com/ee/administration/integration/plantuml.html.

For some reason, I had trouble getting this to work on the GDK, but it worked fine on Omnibus.

1. Run docker run -d --name plantuml -p 8080:8080 plantuml/plantuml-server:tomcat
2. In /admin, configure PlantUML to use http://localhost:8080/plantuml
3. In Procfile, add PLANTUML_ENCODING=deflate to env
4. gitlab-ctl restart rails-web
5. Create an issue with PlantUML text

```plantuml
User -> (Start)
User --> (Use the application) : Label