marathon-slack - update example for 1.11

[judithpatudith]

https://github.com/dcos/examples/tree/master/marathon-slack

Pick any open and unassigned issue in the list of GitHub issues and assign it to yourself by leaving a comment here in the issue. Then update the example by following these steps:

• [ ] Fork the examples repo on GitHub.
• [ ] On a new branch, make a copy of latest example directory (mostly 1.10 or 1.9) for your package and name it 1.11.
• [ ] Test the example on a DC/OS 1.11 cluster. As you test, replace the old screenshots with new ones and make any necessary corrections to the example.
• [ ] Feel free to also improve the instructions.
• [ ] Add a link to your newly created 1.11 example to the README.md file for your package's directory.
• [ ] Report your results:
  • If the example works out of the box, or you are able to correct it, open a PR with your changes (including the example itself and its addition to the README.md file) and add it to the issue.
  • If you can’t get your example to work, add a detailed description of what went wrong on the issue (you still get stickers!).

When you’re done, email [email protected] with the following information: Subject: 1.11 Example Testing - [Service Name] Body:

• Your name
• GitHub Issue link
• Your Twitter handle, if you have one
• Community Slack handle if you have one
• Mailing address (for the stickers)

The Community team shepherd will review your PR and either work with you to make modifications or just merge it if it looks good.