Change to using trunk for this repo

[jeffpaul]

Description of the Change

This PR updates previous master branch references to trunk.

Alternate Designs

n/a

Benefits

Aligns to 10up's "inclusivity in code".

Possible Drawbacks

Any prior references to master will need to be updated to trunk.

Verification Process

Manually verified via GitHub Desktop UI.

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

Relates to #154 and 10up/Open-Source-Best-Practices#62.

Changelog Entry

### Changed
- Renamed `master` branch to `trunk`, please update any references you have accordingly.

[jeffpaul]

@ericmann this PR should cover all references of master in the code base, up to you on how long to let master stick around before you fully remove it. Also, we tend to set our default branch to develop and merge develop to trunk and tag on trunk for releases. Up to you if you want to do that or stick with default branch and release branch both as trunk.