Suggest backend libraries instead of requiring them.
Suggest backend libraries instead of requiring them.

This allows users of the PECL backend to avoid installing unnecassary
third party packages. These packages consume time and bandwidth to
download and deploy across production servers.

The suggestion still makes it clear to other users that they can
continue using the other backends by installing the suggested
packages.

smuuf/celery-php,smuuf/celery-php,smuuf/celery-php