Build expiration date

Add the buildExpiration config and add it to the renderer's config
object. Use grunt to write the build expiration to
config/local-production.json which will override the default value (no
expiration) in production. Finally, run this grunt task as part of the
build process.

// FREEBIE

config/default.json

@@ -1,4 +1,5 @@
 {
   "disableAutoUpdate": false,
-  "openDevTools": false
+  "openDevTools": false,
+  "buildExpiration": 0
 }