Update for sprockets/master context class api

actionpack/lib/sprockets/railtie.rb

@@ -34,7 +34,15 @@ module Sprockets
       app.assets = asset_environment(app)
 
       ActiveSupport.on_load(:action_view) do
-        app.assets.context.instance_eval do
+        if app.assets.respond_to?(:context_class)
+          context = app.assets.context_class
+
+        # TODO: Remove this condition when Sprockets 2.0.beta.3 is released
+        else
+          context = app.assets.context
+        end
+
+        context.instance_eval do
           include ::ActionView::Helpers::SprocketsHelper
         end
       end