Bump webpacker from 5.3.0 to 5.4.0

Bumps [webpacker](https://github.com/rails/webpacker) from 5.3.0 to 5.4.0.
- [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/v5.3.0...v5.4.0)

---
updated-dependencies:
- dependency-name: webpacker
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -360,7 +360,7 @@ GEM
     letter_opener (1.7.0)
       launchy (~> 2.2)
     libv8-node (15.14.0.1)
-    loofah (2.9.1)
+    loofah (2.10.0)
       crass (~> 1.0.2)
       nokogiri (>= 1.5.9)
     mail (2.7.1)
@@ -438,7 +438,7 @@ GEM
       rack (>= 1.2.0)
     rack-protection (2.1.0)
       rack
-    rack-proxy (0.6.5)
+    rack-proxy (0.7.0)
       rack
     rack-rewrite (1.5.1)
     rack-ssl (1.4.1)
@@ -658,7 +658,7 @@ GEM
       addressable (>= 2.3.6)
       crack (>= 0.3.2)
       hashdiff (>= 0.4.0, < 2.0.0)
-    webpacker (5.3.0)
+    webpacker (5.4.0)
       activesupport (>= 5.2)
       rack-proxy (>= 0.6.1)
       railties (>= 5.2)