Update config `form.row_wrapper_class` default value to prevent a float bug

With the previous configuration, `{{ form_row(form.field) }}` (for a "horizontal" form) was generating several `col-*-*` without a `row` to wrap them around. This was causing weird layout bugs.
Merge pull request #564 from lemoinem/patch-1

Update config `form.row_wrapper_class` default value to prevent a row/col bug