Django < 1.11 is no longer supported by Django or Haystack
Test against latest versions of Django 1.11, 2.0, 2.1.
Test against python 2.7 (django 1.11 only), 3.5, 3.6, 3.7 (on xenial).
Update xapian old-stable to 1.2.25, only for py27.
Update xapian old-dev to 1.3.6, only for py27, py34.
Update xapian stable to 1.4.9.
xapian 1.3.6 included just to keep coverage numbers up.
xapian 1.3.7 not included because it didn't build with install-xapian.sh.
This commit is supposed to be reverted at some point: it monkey
patches the whole module test_haystack/__init__.py so Django
is not setup twice.
This was the only way to avoid circular imports when using
django-admin.py to run tests in Django-Haystack.