add "clean" task

build.gradle

@@ -28,6 +28,10 @@ allprojects {
     }
 }
 
+task clean(type: Delete) {
+    delete rootProject.buildDir
+}
+
 task wrapper (type:Wrapper) {
     gradleVersion = '4.4.1'
     distributionUrl = "https://services.gradle.org/distributions/gradle-${gradleVersion}-bin.zip"