Magento 2.0.18 is the final 2.0.x release. After March 2018, Magento 2.0.x will no longer receive security patches, quality fixes, or documentation updates.
To maintain your site's performance, security, and PCI compliance, upgrade to the latest version of Magento.

Cannot run 'composer install'

Suggestion

Change to the directory in which you installed Composer and enter the following command:

mv composer.phar /usr/local/bin/composer

If you have issues with authenticating with the Magento 2 GitHub repository, see Get your authentication keys.