AsciiDoc content & config improvements

- upgrade Asciidoctor Gradle plugin to 1.5.2
- upgrade Gradle to 2.2.1
- fix header in index.asc
- use project version in header
- switch README to AsciiDoc
- fix broken callout number
This configuration tells Asciidoctor to only convert the index file. Otherwise, you get a bunch of warnings in the console as it tried to convert the fragments out of context.