Fix reveal.js Example
Fix reveal.js Example

- Update to latest reveal.js
- Update theme to black (the new default)

Fixes gh-10

rwinch/asciidoctor-gradle-examples,achimgrimm/asciidoctor-gradle-examples,sputnik27/asciidoctor-gradle-examples,sputnik27/asciidoctor-gradle-examples,ysb33r/asciidoctor-gradle-examples,ysb33r/asciidoctor-gradle-examples,achimgrimm/asciidoctor-gradle-examples,rwinch/asciidoctor-gradle-examples