Scheduled monthly dependency update for August

[pyup-bot]

Update celery from 4.2.0 to 4.2.1.

Changelog

4.2.1

=====
:release-date: 2018-07-18 11:00 AM IST
:release-by: Omer Katz

- **Result Backend**: Fix deserialization of exceptions that are present in the producer codebase but not in the consumer codebase.

Contributed by **John Arnold**

- **Message Protocol Compatibility**: Fix error caused by an invalid (None) timelimit value in the message headers when migrating messages from 3.x to 4.x.

Contributed by **Robert Kopaczewski**

- **Result Backend**: Fix serialization of exception arguments when exception arguments are not JSON serializable by default.

Contributed by **Tom Booth**

- **Worker**: Fixed multiple issues with rate limited tasks

Maintain scheduling order.
Fix possible scheduling of a :class:`celery.worker.request.Request` with the wrong :class:`kombu.utils.limits.TokenBucket` which could cause tasks' rate limit to behave incorrectly. 
Fix possible duplicated execution of tasks that were rate limited or if ETA/Countdown was provided for them.

Contributed by :github_user:`ideascf`

- **Worker**: Defensively handle invalid timelimit header values in requests.

Contributed by **Omer Katz**

Documentation fixes:


 - **Matt Wiens**
 - **Seunghun Lee**
 - **Lewis M. Kabui**
 - **Prathamesh Salunkhe**

Links

• PyPI: https://pypi.org/project/celery
• Changelog: https://pyup.io/changelogs/celery/
• Homepage: http://celeryproject.org
• Docs: https://pythonhosted.org/celery/