Adding tests for PHP 5.6 and WordPress 4.7

[ryanwelcher]

Description of the Change

Alternate Designs

Benefits

Possible Drawbacks

Verification Process

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Changelog Entry

[jeffpaul]

While this doesn't technically need to be aligned to a release, I'm punting this to our next release as it's still WIP and unlikely to be done in the very near future.

[peterwilsoncc]

As Travis has been deprecated in favour of GitHub Actions, I'll close this off.

#875 is a draft PR to switch from WPA to Cypress using GitHub actions.