BUG Fix deleted pages redirecting the CMS

Update behat tests for Mink 1.6 compatibility

Fixes issue with viewing of archived (deleted) pages in the cms preview pane which was breaking behat tests

Relies on https://github.com/silverstripe-labs/silverstripe-behat-extension/pull/47 to be merged first
This is cheating a bit, but otherwise the interface wasn't correctly responding to some of our custom automated actions (selecting values with javascript). It should be ok as long as we prove this works manually.