avoid warnging con composer install

Author: bicf

composer.json

@@ -11,7 +11,7 @@
         }
     ],
     "require": {
-      "yiisoft/yii2": "*"
+      "yiisoft/yii2": "2.0.*"
     },
     "repositories": [
         {
@@ -23,5 +23,10 @@
         "psr-4": {
             "bicf\\securityheaders\\": "src"
         }
+    },
+    "config": {
+        "allow-plugins": {
+            "yiisoft/yii2-composer": true
+        }
     }
 }