Bump Haystack version to avoid Django version conflict

The problem is detailed in #1480. We need to require Haystack > 2.3.1 to
avoid it clashing with our support for Django 1.7.
Fix failing order list test

06c2441 changed the way actions are submitted for on the dashboard's
order list view. This commit updates the test to match the new
behaviour.

I'd also argue that the two test methods test different aspects of the
same thing, so I merged them, which saves us one (relatively slow) test
run.
Resurrect requirements_vagrant.txt as requirements_migrations.txt

The file was removed in af3c381d to remove Vagrant, but it served a
dual purpose as requirements to test our suite of migrations. It's
resurrection will hopefully let Travis pass again.