Fix the error by LogNote.start! on Rails 3

On Rails 3, `Rails.logger` is not kind of `Logger`, and
`ActiveSupport::Logger::SimpleFormatter` is not defined. Therefore, the
error is output to Rails log.

```
Footnotes Log Exception: uninitialized constant ActiveSupport::Logger
```

Class which is correspond to `ActiveSupport::Logger::SimpleFormatter` is
`Logger::SimpleFormatter`, so enable to use it.