Further attempts to fix CI (failing)

I tried a lot of things to fix the CI build. I played with vagrant, but i could not reproduce the errors (staling build before the first test case and error 137). For some reason, it still showed the usage statistics dialog. But i think that is not the problem in Travis, since the usage would stale the build after all tests run, it does not effect the test cases.

I also updated the plugins to work with JDK8, and switched to JDK8 in Travis.

I updated the build-tools because right now we used a stale version which was not installed. This is the only real problem this PR fixes.

So this PR cannot fix the CI, but i think it still holds valuable additions, maybe can be merged, or cherry-picked. I have to say i am really embittered about the CI, but @rgladwell spent three days on that...
I'll rebase this as soon as I get home. The advantage of this PR is letting
the project be compiled with JDK8. Actually I updated the plugins when I
learned out that our older findbugs version only works with Java 7.