Fix deprecation warning due to sass-rails-5.0.4

[sauloperez]

Now, with the upgrade to Rails 4.2.7.1 a new deprecation warning shows up:

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 (2 levels) in <class:Railtie> at /home/pau/dev/timeoverflow/vendor/bundle/ruby/2.3.0/gems/sass-rails-5.0.4/lib/sass/rails/railtie.rb:57)
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 (2 levels) in <class:Railtie> at /home/pau/dev/timeoverflow/vendor/bundle/ruby/2.3.0/gems/sass-rails-5.0.4/lib/sass/rails/railtie.rb:58)

It needs to be fixed before we apply another Rails upgrade.