Fix error
Fix error

```
2017-05-28T20:13:20.436081+00:00 app[worker.2]: 4 TID-n4e40 WARN: NameError: undefined local variable or method `logger' for #<DocMailerMaker:0x0000000471dbe0>
2017-05-28T20:13:20.436138+00:00 app[worker.2]: 4 TID-n4e40 WARN: /app/app/models/doc_mailer_maker.rb:59:in `deliver'
2017-05-28T20:13:20.436139+00:00 app[worker.2]: /app/app/models/user.rb:30:in `subscribe_docs!'
2017-05-28T20:13:20.436140+00:00 app[worker.2]: /app/app/jobs/subscribe_user_to_docs.rb:3:in `perform'
```

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