Describe the bug

The warning below is issued every time a Rail 4 project is built.

DEPRECATION WARNING: Sprockets method `register_engine` is deprecated.
       Please register a mime type using `register_mime_type` then
       use `register_compressor` or `register_transformer`.

The error for Rails 5 is fixed by 553e785 which is in master, but not released.

Is would be very nice if a backport of 3.x version of twitter-bootstrap-rails could be released with this bugfix. Because 4.x is not compatible with Rails 4.