Sorry, my typo!

Because I do not find the way to integration bootstrap3 without check bootstrap3 in simple_form codebase, it'll not be merged!

If you want to use it, may be checkout my [integration_bootstrap3](https://github.com/zlx/simple_form/tree/feature/integration_bootstrap3) or [simple_form_bootstrap3](https://github.com/zlx/simple_form_bootstrap3)