Don't cache service worker file

BlakeWilliams · BlakeWilliams · commit af8127aed45c · 2018-02-13T11:39:58.000-05:00
facebook/create-react-app#2440 This should resolve an issue where `index.html` is cached and pointing at an invalid JavaScript bundle.
diff --git a/public/_headers b/public/_headers
@@ -0,0 +1,3 @@
+/service-worker.js
+  Cache-Control: no-cache
+  Max-Age: 0
