Added yoast/phpunit-polyfills for WordPress unit testing updates

Author: rfmeier

composer.json

@@ -13,7 +13,8 @@
 	},
 	"require-dev": {
 		"wp-coding-standards/wpcs": "^1",
-		"phpunit/phpunit": "^6"
+		"phpunit/phpunit": "^6",
+		"yoast/phpunit-polyfills": "^1.0"
 	},
 	"scripts": {
 		"phpcs": "phpcs --standard=src/ruleset-wordpress.xml wpengine-phpcompat.php load-files.php src/*.php",