3. We should exclude the artifacts installed as part of our builds from the cache

It will get easier with the groupId change. But we can probably make it work without too much work if we install only the checkstyle configuration.

4. Upgrade to a more recent version of ubuntu?

Do you have information about this? AFAIK the trusty environment is still in beta and is the most recent Linux OS we can have. We are totally bleeding edge .

5. Maybe use OpenJDK instead of OracleJDK?

I think most people still use the Oracle JDK so I would prefer keeping the tests running on it.