Fix tests for Sprockets 3

Sprockets 3 supports both `register_transformer` and `register_engine` unfortunately they don't play well with one another i.e. if a "transformer" is registered using `register_engine` it behaves slightly differently than when registered via `register_transformer`. This cause problems because the default engines in `lib/sprockets.rb` that are registered all use `register_engine` so if we want to over-write the existing sass engine we have to do both calls when possible.

Author: schneems

lib/sassc/rails/railtie.rb

@@ -53,7 +53,9 @@ class Railtie < ::Rails::Railtie
         if env.respond_to?(:register_transformer)
           env.register_transformer 'text/sass', 'text/css', SassC::Rails::SassTemplate.new #->() { puts "yoyoyoy" }
           env.register_transformer 'text/scss', 'text/css', SassC::Rails::ScssTemplate.new #->() { puts "yoyoyoy" }
-        else
+        end
+
+        if env.respond_to?(:register_engine)
           env.register_engine '.sass', SassC::Rails::SassTemplate
           env.register_engine '.scss', SassC::Rails::ScssTemplate
         end