Adds compatibility for sprockets 2, 3, and 4

Fixes #122

lib/slim-rails/register_engine.rb

@@ -1,6 +1,12 @@
 module Slim
   module Rails
     module RegisterEngine
+      class Transformer
+        def self.call(input)
+          Slim::Template.new(input[:name]) { input[:data] }.render(nil)
+        end
+      end
+
       def self.register_engine(app, config)
         if ::Rails::VERSION::MAJOR == 3
           _register_engine3(app)
@@ -22,7 +28,16 @@ def self._register_engine(config)
         return unless config.respond_to?(:assets)
 
         config.assets.configure do |env|
-          env.register_engine('.slim', Slim::Template)
+          if env.respond_to?(:register_transformer)
+            env.register_mime_type 'text/slim', extensions: ['.slim', '.slim.html']#, charset: :html
+            env.register_preprocessor 'text/slim', 'text/html', RegisterEngine::Transformer
+          end
+
+          if env.respond_to?(:register_engine)
+            args = ['.slim', Slim::Template]
+            args << { silence_deprecation: true } if Sprockets::VERSION.start_with?("3")
+            env.register_engine(*args)
+          end
         end
       end
     end