Diverge 8.1.x-compatible and 8.2.x-compatible versions

Follow-up to #2817299: Diverge 8.1.x-compatible and 8.2.x-compatible versions

Drupal 8.2.0 is out, and there are API changes in core which require us to have different migrate_tools versions - one specific to 8.1.x, and one for 8.2.x (and hopefully above). Cut the 8.x-3.x branch and update the version dependencies appropriately.

I'll try to catch up on at least some existing issues which apply to both first.