This only causes autoload_namespaces.php to work if I add require to the object.

composer.json

{
    "name": "drupal/behat_editor_saucelabs",
    "type": "drupal-module",
    "description": "Runs behat tests on saucelabs.",
    "homepage": "https://github.com/alnutile/behat_editor_saucelabs",
    "license": "GPL-2.0",
    "authors": [
        {
            "name": "Alfred Nutile",
            "email": "[email protected]",
            "role": "The guy behind the keyboard"
        }
    ],
    "support": {
        "issues": "https://github.com/alnutile/behat_editor_saucelabs/issues"
    },
    "require": {
        "behat/behat": "2.4.*@stable"
    },
    "autoload": {
        "psr-0": {
            "Drupal\\BehatEditorSauceLabs": "src/"
        }
    }
}