2.0

• upgrade to newest PMD version 6.13.0 to enable Kotlin support (#38)
• fix wrongly displayed WARNING that cpdCheck is not part of the task graph (#37)
• Use the java-gradle-plugin to generate the plugin descriptors (#40)
• migrate main code to Java 8 as new minor version and upgrade minimal Gradle version (also #32, #35, #41)

Maven Central: 2.0
Gradle Plugins: 2.0