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/serveJavaScriptModule.js

@@ -40,7 +40,7 @@ function serveJavaScriptModule(req, res) {
       .set({
         "Content-Length": Buffer.byteLength(code),
         "Content-Type": getContentTypeHeader(req.entry.contentType),
-        "Cache-Control": "public,max-age=31536000", // 1 year
+        "Cache-Control": "public, max-age=31536000, immutable", // 1 year
         ETag: etag(code),
         "Cache-Tag": "file,js-file,js-module"
       })