ext-bcmath is a requirement of php-amqplib

I started getting an odd error on an old CentOS box when I installed PHP 5.4 on it (described here: https://github.com/tnc/php-amqplib/issues/14). Turns out my installation of PHP 5.4 didn't have php54w-bcmath installed by default, so I installed it and it fixed this issue. The issue would've been easier to discover if php-amqplib's composer.json required the extension ;)