Add branch for MacOS 10.13 when checking 'date' command

[NightFlyer]

Before submitting a pull request, please make sure the following is done:

• [x] Fork the repository and create your branch from master.
• [ ] If you've added code that should be tested, add tests!
• [x] Ensure the test suite passes. It does now that the harness correctly processes dates on Mac OS 10.13!
• [ ] Format your code with ShellCheck.
• [X] Include a human-readable description of what the pull request is trying to accomplish. Fix test harness for MacOS 10.13 so that date tests work correctly.
• [X] Steps for the reviewer(s) on how they can manually QA the changes. Run on MacOS 10.13
• [X] Have a fixes #XX reference to the issue that this pull request fixes. fixes #277

[eightyfive]

I did a smaller (and more future prone) fix FYI: https://github.com/todotxt/todo.txt-cli/pull/291