Add silence_deprecation option to #register_engine

[rosylilly]

Silence deprecation warnings.

ref: https://github.com/rails/sprockets/blob/master/guides/extending_sprockets.md#supporting-all-versions-of-sprockets-in-processors

[bobmaerten]

You should look for @schneems changes on sass-rails
https://github.com/rails/sass-rails/pull/382/files

[schneems]

I actually made the PR that added those register_engine calls in this lib, if you blame the file it goes back to me.

This change is needed to get Sprockets 3.7.0+ to run with no deprecations. I released 3.7.0 after I made this change to sassc-ruby.

The reason this PR is failing is that Sprockets 2 (which this gem supports) doesn't accept a hash as a third argument. You can see the docs for supporting Sprockets 2, 3, and 4 here https://github.com/rails/sprockets/blob/master/guides/extending_sprockets.md#registering-all-versions-of-sprockets-in-processors

It's a little cleaner than my PR to sass-rails.

[bolandrm]

Closing - resolved by #76 - thanks for the PR.