Make compatible custom menu widget for Twitter Bootstrap

Filter 'wp_nav_menu_args'.

add_filter( 'wp_nav_menu_args', 'wpse_79901_nav_menu' );

function wpse_79901_nav_menu( $args )
{
    $args['items_wrap'] = '<ul class="nav nav-pills">%3$s</ul>';
    return $args;
}