Mv `super` call to always return in self.inherited

* This commit moves the return value of thincloud.resque.mailer#self.inherited
  outside of the unless block, ensuring that the subclass will always be passed
  through.

* The reason for this commit was noticed in a client application where devise is
  used for automated password reset via email. Errors of the type below were
  observed:

```
'undefined method `main_app' for #<Class ...>
```

Closes #14