sw.js should be served with max-age=0 (#355)

This is a new requirement for Mastodon v1.5+; it's mentioned here: https://github.com/tootsuite/mastodon/releases/tag/v1.5.0rc1

Running-Mastodon/Production-guide.md

@@ -241,6 +241,11 @@ server {
     try_files $uri @proxy;
   }
   
+  location /sw.js {
+    add_header Cache-Control "public, max-age=0";
+    try_files $uri @proxy;
+  }
+
   location @proxy {
     proxy_set_header Host $host;
     proxy_set_header X-Real-IP $remote_addr;