Test for specific South version as well

See issue #482

Signed-off-by: Michal Čihař <michal@cihar.com>
Translated using Weblate (Azerbaijani)

Currently translated at 39.4% (395 of 1002)
Translated using Weblate (Azerbaijani)

Currently translated at 28.5% (4 of 14)
Merge remote-tracking branch 'origin/master'
Document that we require South 0.7.5

Earlier versions seem to fire post_migrate signal even when the given
migration is not complete, what leads to executing current code on not
yet fully migrated database, what obviously fails.

Fixes #482

Signed-off-by: Michal Čihař <michal@cihar.com>
Translated using Weblate (Azerbaijani)

Currently translated at 50.6% (508 of 1002)
Translated using Weblate (Simplified Chinese)

Currently translated at 90.4% (906 of 1002)
Merge remote-tracking branch 'origin/master'
Revert "Test for specific South version as well"

This reverts commit 3d789f91e1198f09b29a561333ce4da43f15c1f4.
Merge branch 'master' into bootstrap