Generate and attach dependency-tree

Update spring-boot-versions to generate a dependency-tree file and
attach it as an artifact. The file is generated by creating a temporary
POM and calling the invoker plugin.

The spring-boot-versions POM now depends on all spring-boot-starter-*
POMs to ensure that they have been installed before the dependency
tree is processes.

See gh-1047

spring-boot-starters/pom.xml

@@ -20,6 +20,7 @@
 		<main.basedir>${basedir}/..</main.basedir>
 	</properties>
 	<modules>
+		<!-- If you add a starter remember to also add a dependency in spring-boot-versions -->
 		<module>spring-boot-starter</module>
 		<module>spring-boot-starter-amqp</module>
 		<module>spring-boot-starter-aop</module>