always upgrade to 'current' from 'upgrade_version'

The basho builders have defined versions 'current', 'previous', and 'legacy'.
Instead of the odd configuration in the previous commit, upgrade from
'previous' to 'current' by default, and allow the test runner to use the
'upgrade_version' metadata to specify 'legacy' (or anything else) instead
of 'previous'. (thanks @jaredmorrow)