Use stable version of Symfony

Author: Nyholm

composer.json

@@ -28,7 +28,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.0|^5.0",
         "cache/cache": "dev-master",
-        "symfony/cache": "dev-master",
+        "symfony/cache": "^3.1",
         "tedivm/stash": "dev-master"
     },
     "autoload":    {