The test verifies that joining mixed-versions of nodes works in either
direction (old version to new, or new to old) as well as verifies that
handoff works in mixed version clusters.
The motivation for this test was a noted regression in mixed-version
handoff when joining a Riak 1.2 node to an older node (joining in the
opposite direction worked fine).
Related bug fix: basho/riak_core#203