Update m140501_075311_add_oauth2_server.php
Update m140501_075311_add_oauth2_server.php

I was getting the following exception:

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PHP Strict Warning 'yii\base\ErrorException' with message 'Declaration of m140501_075311_add_oauth2_server::primaryKey() should be compatible with yii\db\Migration::primaryKey($length = NULL)'
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Filsh/yii2-oauth2-server,Alegzander/yii2-oauth2-server