Extract old_cluster_keys from changed_keys methods

Allows to deal with partial redis redistribution (when not all of the previous
keys need to be resharded) problems in an easier way (overridding a
single method).

This is basically an internal issue, that I am solving (huge redis with all kinds of functionality on top, being split into a general purpose redis and a distributed array of high performance redis'es), though the change seems small and would allow easier/less intrusive monkey patching for other custom cases.

Also, could You bump the gem version one more time, since the currently released version has a bug in the native migrator.

Thanks