Remove the error occurred by `Footnotes::Notes::LogNote.start!` on Rails 3

On Rails 3, `Rails.logger` is not kind of `Logger`, and
`ActiveSupport::Logger::SimpleFormatter` is not defined. Therefore, this
line is output to Rails log.

```
Footnotes Log Exception: uninitialized constant ActiveSupport::Logger
```

Use `Logger::SimpleFormatter` as it is the class correspond to
`ActiveSupport::Logger::SimpleFormatter`.