Add allow-plugins to composer.json

Author: Atanas Vasilev

composer.json

@@ -25,5 +25,10 @@
         "php": ">=5.3",
         "composer/installers": "^1.0|^2.0",
         "htmlburger/carbon-fields": "3.6.3"
+    },
+    "config": {
+        "allow-plugins": {
+            "composer/installers": true
+        }
     }
-}
+}