Polish exclusions

You mentioned on https://github.com/spring-projects/spring-security/pull/79 

> There appears to be a bug in Gradle which means that some exclusions applied to an artifact do not work reliably. To work around this problem it has been necessary to apply some exclusions at the configuration level

I believe the updates I have made will allow the exclusions to be on distinct dependencies. This has the advantage of cleaning up the Maven poms. Can you please confirm this is still what is needed to be done? Otherwise, can you clarify why this won't work? Thanks!