Bump webpacker from 4.0.2 to 4.0.4 (#10882)

Bumps [webpacker](https://github.com/rails/webpacker) from 4.0.2 to 4.0.4.
- [Release notes](https://github.com/rails/webpacker/releases)
- [Changelog](https://github.com/rails/webpacker/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rails/webpacker/compare/v4.0.2...v4.0.4)

Gemfile.lock

@@ -633,7 +633,7 @@ GEM
       addressable (>= 2.3.6)
       crack (>= 0.3.2)
       hashdiff
-    webpacker (4.0.2)
+    webpacker (4.0.4)
       activesupport (>= 4.2)
       rack-proxy (>= 0.6.1)
       railties (>= 4.2)