Update yoast/phpunit-polyfills requirement from ^1.0 to ^2.0

Updates the requirements on [yoast/phpunit-polyfills](https://github.com/Yoast/PHPUnit-Polyfills) to permit the latest version.
- [Release notes](https://github.com/Yoast/PHPUnit-Polyfills/releases)
- [Changelog](https://github.com/Yoast/PHPUnit-Polyfills/blob/2.x/CHANGELOG.md)
- [Commits](Yoast/PHPUnit-Polyfills@1.0.0...2.0.1)

---
updated-dependencies:
- dependency-name: yoast/phpunit-polyfills
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

composer.json

@@ -15,7 +15,7 @@
     "wp-cli/wp-cli": "~2.4",
     "wp-coding-standards/wpcs": "^3.0",
     "php-coveralls/php-coveralls": "*",
-    "yoast/phpunit-polyfills": "^1.0"
+    "yoast/phpunit-polyfills": "^2.0"
   },
   "config": {
     "bin-dir": "bin",