title: successful install of composer, but message shows: The HOME or COMPOSER_HOME environment variable must be set for composer to run correctly
body: <p>I have installed composer on a Ubuntu vagrant box, running <code>php 7.0</code> (which was just installed prior to this).  Trying simply <code>composer</code>, or the full path <code>php /usr/local/bin/composer</code> both result in the following error:</p>
  
  <pre><code>The HOME or COMPOSER_HOME environment variable must be set for 
  composer to run correctly
  </code></pre>
  
  <p>I am unfamiliar with which env variable to set (or both), what value it should be set to, or where it should be set/declared!  I've searched quite a few forums including github, but I'm not seeing this information.  Appreciate your help.</p>
  
  <p><a href="https://i.stack.imgur.com/TRnKH.png" rel="noreferrer"><img src="https://i.stack.imgur.com/TRnKH.png" alt="error after composer install"></a></p>
  
  <p><strong>UPDATE:</strong> Also tried this, same message:</p>
  
  <pre><code>curl -sS https://getcomposer.org/installer | php
  </code></pre>