Add layout=THIN to Gradle build

Author: Dave Syer

spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/SpringBootPluginExtension.java

@@ -291,6 +291,8 @@ enum LayoutType {
 
 		MODULE(new Layouts.Module()),
 
+		THIN(new Layouts.Thin()),
+
 		NONE(new Layouts.None());
 
 		Layout layout;