PHP error: "The zip extension and unzip command are both missing, skipping." : <p>When I run a <code>composer update</code> I get this error message:</p>

<pre><code>Loading composer repositories with package information
Updating dependencies (including require-dev)
    Failed to download psr/log from dist: The zip extension and unzip command are both missing, skipping.
The php.ini used by your command-line PHP is: /etc/php/7.0/cli/php.ini
    Now trying to download from source
</code></pre>

<p>What do I need to do to enable the zip and unzip commands so that composer can download dependencies?</p>
