Using 'doLast' instead of '<<' for clean task in docs.

docs/build.gradle

@@ -37,8 +37,10 @@ task docs(type: Exec, dependsOn: [copySphinxConf, javadoc]) {
   commandLine 'make', 'html'
 }
 
-task clean << {
+task clean {
+  doLast {
     ant.delete dir: 'build'
     ant.delete file: 'src/main/sphinx/conf.py'
   }
+}