Use "immutable" Cache-Control extension

See http://bitsup.blogspot.com/2016/05/cache-control-immutable.html for
more info.

Fixes #103
Closes #119

modules/middleware/fetchPackage.js

@@ -92,8 +92,8 @@ function filenameRedirect(req, res) {
   // and URLs resolve correctly.
   res
     .set({
-      "Cache-Control": "public,max-age=31536000", // 1 year
-      "Cache-Tag": "redirect,filename-redirect"
+      "Cache-Control": "public, max-age=31536000, immutable", // 1 year
+      "Cache-Tag": "redirect, filename-redirect"
     })
     .redirect(
       302,