Fixes #112

The fix is pretty simple, but there is no associated unittest. I have manually tested it by using the gradle scirpt as shown in #112.

The reason I have no unittest is due to the present testCompile `org.asciidoctor:asciidoctorj:1.5.0` line in build.gradle. This places `asciidoctorj` on the classpath, rendering half of the `setupClassloader` function during unit testing useless. In order to reproduce the problem, one needs to run with a test configuration that does not have `asciidoctorj` as a dependency. Just removing it does not work as it will break `AsciidoctorTaskSpec`. It was not worth the effort at present.

The fix itself it pretty simple - when the task action is executed and the (not so aptly named) `classpath` property is null, then set it to `project.configurations.asciidoctor`.