WordPress 3.8 – Dashboard 1 Column Screen Options

Another solution is to add the columns setting back to the settings.

function wpse126301_dashboard_columns() {
    add_screen_option(
        'layout_columns',
        array(
            'max'     => 2,
            'default' => 1
        )
    );
}
add_action( 'admin_head-index.php', 'wpse126301_dashboard_columns' );

There is a related Trac ticket: #26354