Fix converters and extensions dependency configuration

[abelsromero]

As seen in https://github.com/asciidoctor/asciidoctor-gradle-plugin/issues/203, the right way to setup converters and extensions is using the asciidoctor configuration in the project's dependencies. Current examples define them as classpath dependencies in the buildscript section, which causes issues running with the gradle daemon.

Currently, the following examples should be fixed:

• asciidoc-to-all-example
• asciidoct-to-epu3-example
• asciidoc-to-pdf-example
• asciidoc-to-pdf-with-theme-example

[abelsromero]

This has already been fixed