feat(gatsby-plugin-netlify) Do not cache service worker file

Related to https://www.netlify.com/blog/2018/06/28/5-pro-tips-and-plugins-for-optimizing-your-gatsby---netlify-site/#4-get-your-service-workers-um-working

Recommandation is not to cache service worker file : w3c/ServiceWorker#893

packages/gatsby-plugin-netlify/src/constants.js

@@ -26,6 +26,7 @@ export const SECURITY_HEADERS = {
 
 export const CACHING_HEADERS = {
   "/static/*": [`Cache-Control: public, max-age=31536000, immutable`],
+  "/sw.js": [`Cache-Control: no-cache`],
 }
 
 export const LINK_REGEX = /^(Link: <\/)(.+)(>;.+)/