composer.json

{
    "repositories": [
        {
            "type": "package",
            "package": {
                "name": "wordpress",
                "type": "webroot",
                "version": "4.9.1",
                "dist": {
                    "type": "zip",
                    "url": "https://github.com/WordPress/WordPress/archive/4.9.1.zip"
                },
                "require" : {
                    "fancyguy/webroot-installer": "1.0.0"
                }
            }
        }
    ],
    "require": {
        "wordpress": "4.*",
        "fancyguy/webroot-installer": "1.0.0"
    },
    "extra": {
        "webroot-dir": "public/wp",
        "webroot-package": "wordpress"
    }
}