Upgrade php-amqplib to v2.9

I'm getting the error `stream_select(): unable to select [4]: Interrupted system call (max_fd=6)` on my consumers. This error [was fixed in version 2.9 of php-amqplib](https://github.com/php-amqplib/php-amqplib/issues/670) but [php-enqueue uses version 2.8](https://github.com/php-enqueue/amqp-lib/blob/master/composer.json).