Allow including the same version of symfony/yaml as Drupal core

Author: lammensj

composer.json

@@ -11,7 +11,7 @@
     "require": {
         "php": "^7.1",
         "phpstan/phpstan": "^0.10.6",
-        "symfony/yaml": "^4.2"
+        "symfony/yaml": "~3.4.5|^4.2"
     },
     "autoload": {
         "psr-4": {