Prepare for Gradle 6 upgrade

Remove/replace
 * deprecated "IvyArtifactRepository.layout(String, Closure)" and
 * unnecessary overwrite: true - we don't replace existing tasks.
diff --git a/build.gradle b/build.gradle
index 2e820a8..c77945d 100644
--- a/build.gradle
+++ b/build.gradle
@@ -499,7 +499,7 @@
         // Gemini Web and Tomcat dependencies aren't at Maven Central
         ivy {
             url "https://build.eclipse.org/rt/virgo/ivy/bundles/release/"
-            layout "pattern", { artifact "[organisation]/[module]/[revision]/[artifact]-[revision].[ext]" }
+            patternLayout { artifact "[organisation]/[module]/[revision]/[artifact]-[revision].[ext]" }
         }
     }
 
diff --git a/buildSrc/src/main/groovy/aspectj/AspectJPlugin.groovy b/buildSrc/src/main/groovy/aspectj/AspectJPlugin.groovy
index 4f80707..2ba0695 100644
--- a/buildSrc/src/main/groovy/aspectj/AspectJPlugin.groovy
+++ b/buildSrc/src/main/groovy/aspectj/AspectJPlugin.groovy
@@ -46,7 +46,7 @@
     void setupAspectJ(Project project) {
         project.tasks.withType(JavaCompile) { javaCompileTask ->
             def javaCompileTaskName = javaCompileTask.name
-            def ajCompileTask = project.tasks.create(name: javaCompileTaskName + 'Aspect', overwrite: true, description: 'Compiles AspectJ Source', type: Ajc) {
+            def ajCompileTask = project.tasks.create(name: javaCompileTaskName + 'Aspect', description: 'Compiles AspectJ Source', type: Ajc) {
                 inputs.files(javaCompileTask.inputs.files)
                 inputs.properties(javaCompileTask.inputs.properties.findAll {it.value != null})