"Use" Command with CI?

I've used npm a lot for node.js, but this is my first time using Composer (with PHP of course). Here is my current composer.json file, (which is located in the same directory as index.php):

Code:

{
        "require": {
            "jonnyw/php-phantomjs": "4.*"
        },
        "config": {
            "bin-dir": "bin"
        },
        "scripts": {
            "post-install-cmd": [
                "PhantomInstaller\\Installer::installPhantomJS"
            ],
            "post-update-cmd": [
                "PhantomInstaller\\Installer::installPhantomJS"
            ]
        }
    }

What is the correct way to modify this file so as to turn on composer?