Remove reference to BlankSlate

This has actually been removed from the builder gem in rails/builder#15
Additionally, `BasicObject` will be more familiar to users.

Author: Earlopain

activesupport/lib/active_support/proxy_object.rb

@@ -3,8 +3,8 @@
 module ActiveSupport
   # = Active Support Proxy \Object
   #
-  # A class with no predefined methods that behaves similarly to Builder's
-  # BlankSlate. Used for proxy classes.
+  # A class with no predefined methods that behaves similarly to Ruby's
+  # BasicObject. Used for proxy classes.
   class ProxyObject < ::BasicObject
     undef_method :==
     undef_method :equal?