Rq

* Reverted changes to `hmset` command which causes workers on Redis server < 4 to crash. Thanks selwin!
* Merged in more groundwork to enable jobs with multiple dependencies. Thanks thomasmatecki!

* Default serializer now uses `pickle.HIGHEST_PROTOCOL` for backward compatibility reasons. Thanks bbayles!
* Avoid deprecation warnings on redis-py >= 3.5.0. Thanks bbayles!

* Custom serializer is now supported. Thanks solababs!
* `delay()` now accepts `job_id` argument. Thanks grayshirt!
* Fixed a bug that may cause early termination of scheduled or requeued jobs. Thanks rmartin48!
* When a job is scheduled, always add queue name to a set containing active RQ queue names. Thanks mdawar!
* Added `--sentry-ca-certs` and `--sentry-debug` parameters to `rq worker` CLI. Thanks kichawa!
* Jobs cleaned up by `StartedJobRegistry` are given an exception info. Thanks selwin!
* Python 2.7 is no longer supported. Thanks selwin!

- Run `rqworker` with `--sentry_dsn=""` to disable Sentry integration. Thanks Bolayniuss!
- Support for `SSL` Redis kwarg. Thanks ajknv!
- `rqworker`and `rqscheduler` management commands now uses RQ's built in `setup_loghandlers` function. Thanks Paulius-Maruska!
- Remove the use of deprecated `admin_static` template tag. Thanks lorenzomorandini!

- Added support `redis-py` >= 3 and `RQ` >= 0.13. Thanks selwin!
- Use `Worker.count(queue=queue)` to speed up the process of getting the number of active workers. Thanks selwin!
- Added an option to requeue job from the admin interface. Thanks seiryuz!
- Improve Sentinel support. Thanks pnuckowski!

* `Job.fetch()` now properly handles unpickleable return values. Thanks selwin!