Remove "immutable" from long-term responses

We had a bug recently which sent out some bad "immutable" responses. We
need to have a better process around cutting releases before we can be
sure it doesn't happen again.

modules/actions/serveJavaScriptModule.js

@@ -14,7 +14,7 @@ function serveJavaScriptModule(req, res) {
       .set({
         'Content-Length': Buffer.byteLength(code),
         'Content-Type': getContentTypeHeader(req.entry.contentType),
-        'Cache-Control': 'public, max-age=31536000, immutable', // 1 year
+        'Cache-Control': 'public, max-age=31536000', // 1 year
         ETag: etag(code),
         'Cache-Tag': 'file, js-file, js-module'
       })