[ticket/11588] Also use version.phpbb.com in install_update.php.

PHPBB3-11588
Merge remote-tracking branch 'remotes/bantu/ticket/11588' into develop-olympus

# By Andreas Fischer
# Via Andreas Fischer
* remotes/bantu/ticket/11588:
  [ticket/11588] Also use version.phpbb.com in install_update.php.