Define Version range for yaml component to keep compatibility with symfony 2.7-3.x

Author: Marc Aschmann

composer.json

@@ -17,7 +17,7 @@
   ],
   "require": {
     "php": ">=5.4.0",
-    "symfony/yaml": "^3.1"
+    "symfony/yaml": ">=2.7 <=4.0"
   },
   "require-dev": {
     "phpunit/phpunit": "<5.0",