用composer 安装laravel 5出现 PHP Fatal error: Class 'Illuminate\Foundation\Application' not found in

按照官方文档上面的安装方法。

在命令行执行这条命令
composer create-project laravel/laravel --prefer-dist

把所有包都下载完成后，出现这个错误：

Generating autoload files
PHP Fatal error: Class 'Illuminate\Foundation\Application' not found in /data/wwwroot/laravel/bootstrap/app.php on line 14

Fatal error: Class 'Illuminate\Foundation\Application' not found in /data/wwwroot/laravel/bootstrap/app.php on line 14
Script php artisan clear-compiled handling the post-install-cmd event returned with an error: PHP Fatal error: Class 'Illuminate\Foundation\Application' not found in /data/wwwroot/laravel/bootstrap/app.php on line 14

Fatal error: Class 'Illuminate\Foundation\Application' not found in /data/wwwroot/laravel/bootstrap/app.php on line 14
PHP Fatal error: Class 'Illuminate\Foundation\Application' not found in /data/wwwroot/laravel/bootstrap/app.php on line 14
Script php artisan optimize handling the post-install-cmd event returned with an error: PHP Fatal error: Class 'Illuminate\Foundation\Application' not found in /data/wwwroot/laravel/bootstrap/app.php on line 14

环境：
系统：mac os x 10.10.3
php：5.6.7（已安装mcrypt）
nginx+fpm