Update Rollup to use HMR with Sapper

src/server.js

@@ -3,10 +3,12 @@ import polka from 'polka'
 import compression from 'compression'
 import * as sapper from '@sapper/server'
 
+// Define environment
 const { PORT, NODE_ENV } = process.env
 const dev = NODE_ENV === 'development'
 
-polka() // You can also use Express
+// Setup server
+polka()
 	.use(
 		compression({ threshold: 0 }),
 		sirv('static', { dev }),