FIX: use newer versions of CVMFS for test 508
Merge pull request #214 from reneme/fix_test_508

HOTFIX: Testcase 508 uses available versions of cvmfs source tar balls

This works around the loss of the old cvmfs tar balls.

Note: The test requires 4 tar balls, but at the moment we only have two of them. This test should be adapted to use new versions of cvmfs source tar balls, as soon as they are available.