FIX: Use Diverse CVMFS Tarballs in Test 508

Integration Test 508 uses tarballs of the CVMFS sources to do an incremental update test on the backend system. Since ancient tarballs were lost a while ago, we needed to do this test in a stripped down fashion with only two instead of four different CVMFS tarball versions. Now we have enough history to bring the test back into normal mode.