Engine registered using a deprecated Sprockets syntax

[n0nick]

Running Jade with Sprockets v3.7.0 gives the following deprecation warning:

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
 (called from block in <class:Engine> at $HOME/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/jade-0.1.1/lib/jade/engine.rb:8)