Fixes yaml version for php 5.4

Author: Keith Kirk

composer.json

@@ -31,7 +31,7 @@
         "symfony/finder": "~2.3|^3.0",
         "symfony/filesystem": "~2.3|^3.0",
         "symfony/phpunit-bridge": "^4.0",
-        "symfony/yaml": "^3.4"
+        "symfony/yaml": "~2.8|^3.0"
     },
     "suggest": {
         "aws/aws-sdk-php": "Required to use AWS as a Queue Provider",