Update README.md

Author: mootensai

README.md

@@ -8,7 +8,7 @@ The preferred way to install this extension is through [composer](http://getcomp
 Either run
 
 ```bash
-$ composer require mootensai/yii2-relation-trait
+$ composer require 'mootensai/yii2-relation-trait:dev-master'
 ```
 
 or add