Use "immutable" Cache-Control extension

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

Fixes #103
Closes #119

modules/actions/serveMetadata.js

@@ -35,7 +35,7 @@ function serveMetadata(req, res) {
 
   res
     .set({
-      "Cache-Control": "public,max-age=31536000", // 1 year
+      "Cache-Control": "public, max-age=31536000, immutable", // 1 year
       "Cache-Tag": "meta"
     })
     .send(metadata);