After a recent update to WordPress 4.2.1 I was met with the following once trying to access the admin:
/wp-admin/ would redirect to /wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2F to resolve the issue I downloaded the latest version of WordPress, extracted version.php and manually uploaded the file to /wp-includes/ following this I was then able to update the database and all is well again.