Allow for more recent 2.6.x versions

Inspired by rabbitmq/rabbitmq-website#360.

Author: michaelklishin

php/composer.json

@@ -1,5 +1,5 @@
 {
  "require": {
- "php-amqplib/php-amqplib": "v2.6.1"
+ "php-amqplib/php-amqplib": ">=2.6.1"
  }
 }