Use configure blocks to change the sprockets environment

sprockets-rails 3 will build the sprockets environment only in a
after_initialize block so app.assets is never defined in a railtie
initializer.

lib/sassc/rails/railtie.rb

@@ -44,14 +44,14 @@ class Railtie < ::Rails::Railtie
       #   config.sass.full_exception = app.config.consider_all_requests_local
       # end
 
-      if app.assets
-        app.assets.context_class.class_eval do
+      app.config.assets.configure do |env|
+        env.context_class.class_eval do
           class_attribute :sass_config
           self.sass_config = app.config.sass
         end
 
-        app.assets.register_engine '.sass', SassC::Rails::SassTemplate
-        app.assets.register_engine '.scss', SassC::Rails::ScssTemplate
+        env.register_engine '.sass', SassC::Rails::SassTemplate
+        env.register_engine '.scss', SassC::Rails::ScssTemplate
       end
     end
 

test/sassc_rails_test.rb

@@ -22,7 +22,9 @@ def setup
     @app.config.sass.line_comments    = false
 
     # Add a fake compressor for testing purposes
-    @app.assets.register_compressor "text/css", :test, TestCompressor
+    @app.config.assets.configure do |env|
+      env.register_compressor "text/css", :test, TestCompressor
+    end
 
     Rails.backtrace_cleaner.remove_silencers!
   end