Bump celery-haystack-ng to 2.0 to support batch update tasks

Closes AlekSIS-App-CSVImport#37 (closed)

CHANGELOG.rst

@@ -31,6 +31,7 @@ Fixed
 * The ``Person`` model had an ``is_active`` flag that was used in unclear ways; it is now removed
 * The data check results list view didn't work if a related object had been deleted in the meanwhile.
 * Socialaccount login template was not overriden
+* Atomic transactions now cause only one Haystack update task to run
 
 Changed
 ~~~~~~~