update composer file to fix unit testing

Author: pfefferle

composer.json

@@ -4,7 +4,7 @@
     "type": "wordpress-plugin",
     "require": {
         "php": ">=5.6.0",
-        "composer/installers": "~1.0"
+        "composer/installers": "~2.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^5.7.21 || ^6.5 || ^7.5 || ^8",
@@ -15,6 +15,9 @@
         "yoast/phpunit-polyfills": "^1.0",
         "dealerdirect/phpcodesniffer-composer-installer": "^0.7.1"
     },
+    "allow-plugins": {
+        "composer/installers": true
+    },
     "license": "MIT",
     "authors": [
         {