Updated WordPress Libraries to 1.3.9

[n7studios]

Summary

Updates WordPress Libraries to 1.3.9

Removes localized error messages, as the text domain (convertkit) for this Plugin matches the main ConvertKit Plugin - therefore, the localized error messages supplied by the WordPress Libraries don't need to have their text domain changed.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)