Add slash after dirname for fixture paths - advanced application template

When I attempted to run `codeception/bin/yii_functional fixture user` I received the error message `Error: The template path "/path/to/yii2-app-advanced/tests/codeceptioncommon/templates" not exist`. 

In my tests, `dirname(__DIR__)` does not result in a trailing slash, so I think it's necessary to add the slash to the beginning of the appended path. Same for yii_acceptance and yii_unit. I'm using Ubuntu 14.04 and PHP 5.5.9.

I also suggest a grammar change to the wording of the related error message.