After upgrading to Wordpress 4.4 I started to get a similar error when trying to deploy:
TASK: [deploy | WordPress Installed?] ***************************************** failed: [46.101.217.38] => {"changed": false, "cmd": ["wp", "core", "is-installed"], "delta": "0:00:00.260645", "end": "2015-12-10 21:38:46.873124", "failed": true, "failed_when_result": true, "rc": 255, "start": "2015-12-10 21:38:46.612479", "stdout_lines": [], "warnings": []} stderr: PHP Fatal error: Class 'WP_Widget' not found in /srv/www/staging.example.com/releases/20151210213809/web/wp/wp-includes/widgets/class-wp-widget-pages.php on line 17 Fatal error: Class 'WP_Widget' not found in /srv/www/staging.example.com/releases/20151210213809/web/wp/wp-includes/widgets/class-wp-widget-pages.php on line 17