PB-369: Integrate third party library to work with DOMDocument

 - adjust composer.json

Author: cspruiell

app/code/Magento/PageBuilder/composer.json

@@ -21,9 +21,7 @@
         "magento/module-wishlist": "*",
         "magento/module-require-js": "*",
         "magento/module-media-storage": "*",
-        "php": "~7.1.3||~7.2.0||~7.3.0"
-    },
-    "require-dev": {
+        "php": "~7.1.3||~7.2.0||~7.3.0",
         "phpgt/dom": "^2.1.6"
     },
     "suggest": {