Fix extending ActionView::Base instances

In rails/rails@1165401 the Active Support core extension for
Kernel#class_eval was removed so we need to call it on the
singleton_klass ourselves.

lib/sprockets/rails/helper.rb

@@ -58,7 +58,7 @@ def assets_environment
       end
 
       def self.extended(obj)
-        obj.class_eval do
+        obj.singleton_class.class_eval do
           attr_accessor(*VIEW_ACCESSORS)
 
           remove_method :assets_environment