Proguard in a gradle for a pure Java project

gistfile1.groovy

buildscript {
    repositories {
        jcenter()
    }

    dependencies {
        classpath 'net.sf.proguard:proguard-gradle:5.2'
    }
}

task proguard(type: proguard.gradle.ProGuardTask) {
    configuration 'myconfig.pro'
    libraryjars "${System.getProperty('java.home')}/lib/rt.jar"
}