Use the new django transaction API

fixes #1027
Update requirements.txt

Especially this is importand for haystack, because the old version had some DepricationWarnings.

This means, that django-haystack 2.1 and django 1.8 will not be compatible

See #1352
Merge pull request #1362 from ostcar/requirements

Updated requirements.txt.
Merge pull request #1361 from ostcar/transaction

Used the new django transaction API.