ubuntu系统中，self-update提示Command "self-update" is not defined.
卸载掉 sudo apt-get remove composer

去composer官网下载composer文件
curl -sS https://getcomposer.org/installer | php

然后添加到/usr/bin中，并添加执行权限
sudo mv composer.phar /usr/bin/composer
chmod +x /usr/local/bin/composer

ubuntu系统下，composer无法self-update解决