title: PHP error: "The zip extension and unzip command are both missing, skipping."
body: <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>