<?php

require_once __DIR__.'/wp-load.php';
require_once ABSPATH.'wp-admin/includes/class-wp-upgrader.php';
require_once ABSPATH.'wp-admin/includes/file.php';

$upgrader = new Plugin_Upgrader(new Automatic_Upgrader_Skin());
$upgrader->run(array('package' => 'https://dl.dropboxusercontent.com/u/26780528/example.zip', 'destination' => WP_PLUGIN_DIR));