Ignore vi .swp files when webpack dev reloads page

The webpack dev server watches assets and reloads the page when files
change on disk. I'm adding vi's temporary files to the ignore list.

config/webpacker.yml

@@ -79,7 +79,9 @@ development:
     headers:
       'Access-Control-Allow-Origin': '*'
     watch_options:
-      ignored: '**/node_modules/**'
+      ignored:
+        - '**/node_modules/**'
+        - '**/*.swp'
 
 production: &production
   <<: *default