Fix JDependPlugin task configuration

The JDepend plugin is being too eager to resolve its dependencies. When attempting to run the "tasks" task with the JDepend plugin applied, gradle dies with "Could not determine the dependencies of task ':jdependTest'." (see Forum post http://forums.gradle.org/gradle/topics/applying_the_jdepend_plugin_causes_gradle_tasks_to_fail_with_could_not_determine_the_dependencies_of_task_jdependtest)

This pull request fixes the problem by adopting a configuration similar to that used in the CodeNarc plugin. This permits "gradle tasks" to complete successfully.