Require mbstring

When it is not installed a `PHP Fatal error:  Call to undefined function PhpAmqpLib\Wire\mb_strlen() in /home/stackato/app/vendor/videlalvaro/php-amqplib/PhpAmqpLib/Wire/AMQPReader.php on line 80` is raised.
Merge pull request #214 from Mihailoff/patch-1

Require mbstring