Upgrade asciidoctor-gradle template to Gradle 2.5

[mojavelinux]

Upgrade the asciidoctor-gradle template to Gradle 2.5. The watch plugin can also be removed as this is now covered by the -t flag.

The index.adoc file should also be updated to the following:

= Your Documentation
Your Name <you@example.org>
:toclevels: 1
:sectnums:

Write your content here.

I think that provides a better starting point. It also uses the recommended attribute name for enabling section numbers.