It's easy to test multiple guava versions e.g. mvn clean install -Dguava.version=14.0.1 builds and runs the suite with Guava 14.0.1. Similarly, testing with multiple JDK versions can be scripted.
But testing multiple Druid versions would require tearing down & rebuilding the VM between runs. And the Vagrant script is not even parameterized currently.
I am inclined to just have one supported version of Druid, unless we get some more resources for testing. If people certify an RC on other Druid versions we can add that to the release notes, but it would not be an exit criterion for the release.