adding missing symfony/asset closes #859 (#871)

Author: saimaz

composer.json

@@ -16,6 +16,7 @@
         "symfony/console": "^2.8|^3.0|^4",
         "symfony/stopwatch": "^2.8|^3.0|^4",
         "symfony/templating": "^2.8|^3.0|^4",
+        "symfony/asset": "^2.8|^3.0|^4",
         "doctrine/annotations": "~1.2",
         "doctrine/inflector": "~1.0",
         "doctrine/cache": "~1.4",