Use symfony/lts for 3.3 branch too

Author: ostrolucky

composer.json

@@ -9,6 +9,7 @@
         "symfony/console": "^3.3",
         "symfony/flex": "^1.0",
         "symfony/framework-bundle": "^3.3",
+        "symfony/lts": "^3",
         "symfony/yaml": "^3.3"
     },
     "require-dev": {