Silence deprecation warnings for Sprockets

[leemeichin]

Sprockets will spam logs with this warning (see #72 too):

DEPRECATION WARNING: Sprockets method `register_engine` is deprecated.
Please register a mime type using `register_mime_type` then
use `register_compressor` or `register_transformer`.
https://github.com/rails/sprockets/blob/master/guides/extending_sprockets.md#supporting-all-versions-of-sprockets-in-processors

This is a super quick fix to silence that, since adding fully Sprockets v4 compatibility requires a little extra work.