Update attributes.js

I need to add this print-scaling line to get scaling to work in my ipp implementation.  Can we add it to the master branch so I don't need to manually add it every time I deploy?

lib/attributes.js

@@ -515,6 +515,7 @@ attributes["Job Template"] = {
   "print-quality":                                enumeration,
   "print-rendering-intent":                       keyword,
   "printer-resolution":                           resolution,
+  "print-scaling":                                keyword,
   "proof-print":                                  collection({
     "media":                                      _(keyword, name),
     "media-col":                                  collection("Job Template", "media-col"),