There are some places where releases are only located if they are in the
state DEPLOYED. That particular logic was incorrectly used for upgrades.
That caused #1566. While fixing that issue, I found that this was also
the root cause of #1587 (though because it was off by one). I added a
generic method to get the last release, regardless of its status.

This allows some behaviors that previously failed:

- 'helm upgrade' can now be performed on a DELETED release
- 'helm rollback' can now be performed on a DELETED release even if
  there is only one revision of that release history.

Closes #1566
Closes #1587