allow all versions of simple forms '~> 3.0'

For my app, I need a newer version of simple forms to be compatible with
Bootstrap 3. I've tried out the admin backend, and it still seems to
work (since Alchemy uses its own simple form wrappers).
convert config.label_text from lambda to proc

That way, it can accept two as well as three arguments and does not
throw an error when updating. Copied from
http://github.com/plataformatec/simple_form/commit/c5cd96675ca413c5b9ea7f20bbe61a0410dbd59a
Merge pull request #660 from mamhoff/free-simple-forms

Free simple forms