Cap the django-haystack dependency to avoid 3.0 installs.
Cap the django-haystack dependency to avoid 3.0 installs.

W're now requiring a 2.x release, with 2.7 at the minimum. This is the
last version series to support Python 2.7, which is required currently,
and ensures we're using the same version series for both Python 2 and 3
installs.

chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard