Jobtastic

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* Tiny changes for better ``unicode`` type support.
* Dropping obsolete Celery3.1x from test cicle. It works for current release,
but we wouldn't warrant it in the future.

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* Added ``async_or_eager`` and ``async_or_run`` methods
so that you can do things like modifying the ``queue`` or ``routing_key`` with Jobtastic.
Thanks to thenewguy for the PR and months of persistence!
* Added support for celery 4.

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* Dropped support for Django prior to 1.6
* Dropped support for Celery prior to 3.1
* Dropped support for Python 2.6
* Dropped support for Python < 3.5 - thanks midnightlynx for 3.4/3.5 support
* Support for pluggable caches - thanks rhunwicks
* Fixed `setup.py` license to match MIT license in repo - thanks sodre

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* Switched to the psutil 3.0 API.

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* Dropped support for Django 1.3.
It's no longer getting security updates.

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* The `update_frequency` argument to `update_progress()` wasn't properly
delaying updates to the back end after the first increment. This resulted
in many more progress updates to your backend, which means you should
see performance gains with this release if you took advantage of that option.
Thanks to Ilya Baryshev for the fix.
* Logging calls related to `update_progress()` are now at the `DEBUG` level
instead of `LOG`. Hopefully your logs are much more readable, as a result.
Thanks to Kyle Gibson for convincing me that things used for debugging
should probably be at the debug level.