Update periodic-tasks.rst

Midnight added as an occurance
Merge pull request #1959 from AbsoluteMSTR/patch-1

Update periodic-tasks.rst
Docs: generic celerybeat options should not write schedule to root owned directory.  Thanks to Nikos Fertakis.
Merge branch 'master' of github.com:celery/celery
Add option to worker to control heartbeat interval.

Thanks to my colleague Craig Northway for the patch.
Update task.py
Update tasks.rst
Merge pull request #1970 from dtheodor/master

Fix documentation for Task.throws
Merge branch 'master' of github.com:celery/celery
Merge pull request #1966 from mduggan/worker-heartbeat-option

Add option to worker to control heartbeat interval.
Try to fix CI tests
Make celery.five importable from outside.  Closes #1968
Fixes tests
Fixes test for #1964
Wording
Adds celery.version_info_t
Fix for QoS when using RabbitMQ 3.3 or later.  (Issue celery/kombu#339)
Add my name to contributors
Merge pull request #1983 from mduggan/worker-heartbeat-option

Add my name to contributors
Changelog stub for 3.1.11