Follow sproket engine interface guidelines

[nickd0]

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

new.render on Sprokets engines is deprecated as of Sprokets v3.7.0 (its currently called through the Sprockets::LegacyTiltProcessor class - see sprockets-3.7.0/lib/sprockets/legacy_tilt_processor.rb:20).

This PR implement the class method call which instantiates a CssSplitter::SproketEngine and calls render.

[jhilden]

thanks @nick-donald I'm assuming that this is a fix for #69

could @adzap and @bhalash please try this out and report if it works?

is thery anything that we could/should add a test for?

[jhilden]

i find this syntax a little hard to understand. could this be expressed in a more simple way?

[nickd0]

@jhilden Clarified and added comments.

As stated above this is a fix for #69

I don't think this needs additional tests since this is .call is only every called by Sprokets. And since #evaluate is left intact, it should be fully backwards compatible.

[jhilden]

could we get some feedback from @adzap and @bhalash on this, so I can merge this?

[adzap]

@jhilden looks good. Fixes deprecations.

[oboxodo]

Any plans to finally merge this and release a new gem version? It would be pretty helpful.

[jhilden]

Sorry for the delay, since I'm on vacation. PR is merged an release should follow in the next couple of days. Thanks for testing the PR.