XD-2203 Make sure Spring XD's PDF reference doc has right release revision references

* Add `appversion` attribute under the `asciidoctorDocbook` Gradle task
* Update `asciidoctor-gradle-plugin` to `1.5.0`
* Update `docbook-reference-plugin` to `0.3.0`
* Add `compat-mode` attribute under the `asciidoctorDocbook` Gradle task
* Add `compat-mode` attribute under the `asciidoctorHtml` Gradle task

Jira: https://jira.spring.io/browse/XD-2203
yes - I guess we can remove the line as not presently needed. The plugin sets its *default* version. But by setting `asciidoctorjVersion` you can explicitly specify the `asciidoctorj` version to be used.