Require ^3.4 yaml for testing

Author: Pavel Batanov

composer.json

@@ -20,7 +20,7 @@
   },
   "require-dev": {
     "phpunit/phpunit": "^7.0",
-    "symfony/yaml": "^4.0"
+    "symfony/yaml": "^3.4 || ^4.0"
   },
   "autoload": {
     "psr-4": {