title: Composer fails to self-update
body: <p>When I try to run composer self-update, I get the error:</p>
  
  <pre><code>The file is corrupted (creating archive "/home/heytherejake/.composer/cache/composer-temp.phar" disabled by the php.ini setting phar.readonly).
  Please re-run the self-update command to try again.
  </code></pre>
  
  <p>I have set phar.readonly set to Off (and my phpinfo reflects this), yet the error still occurs. Does anyone know what else might cause this?</p>