Stopped draft3 from registering itself as the default validator

Draft4 also registers itself as default, so which one ends up as default
depends on which one gets loaded first!

lib/json-schema/validators/draft3.rb

@@ -31,8 +31,7 @@ def initialize
       end
 
       JSON::Validator.register_validator(self.new)
-      JSON::Validator.register_default_validator(self.new)
     end
 
   end
-end
+end