perf: avoid caching robots.txt in service worker (#820)

src/service-worker.js

@@ -18,6 +18,7 @@ const assets = __assets__
   .map(file => file.startsWith('/') ? file : `/${file}`)
   .filter(filename => !filename.startsWith('/apple-icon'))
   .filter(filename => !filename.endsWith('.map'))
+  .filter(filename => filename !== '/robots.txt')
 
 // `shell` is an array of all the files generated by webpack
 // also contains '/index.html' for some reason