Bump webpacker from 3.5.5 to 4.3.0

Bumps [webpacker](https://github.com/rails/webpacker) from 3.5.5 to 4.3.0.
- [Release notes](https://github.com/rails/webpacker/releases)
- [Changelog](https://github.com/rails/webpacker/blob/master/CHANGELOG.md)
- [Commits](rails/webpacker@v3.5.5...v4.3.0)

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -101,13 +101,13 @@ GEM
       jekyll (>= 1.3)
       listen (>= 1.3)
       rack (~> 1.5)
-    rack-proxy (0.6.4)
+    rack-proxy (0.6.5)
       rack
     rack-rewrite (1.5.1)
     rack-ssl-enforcer (0.2.9)
     rack-test (0.6.3)
       rack (>= 1.0)
-    rails-deprecated_sanitizer (1.0.3)
+    rails-deprecated_sanitizer (1.0.4)
       activesupport (>= 4.2.0.alpha)
     rails-dom-testing (1.0.9)
       activesupport (>= 4.2.0, < 5.0)
@@ -136,7 +136,7 @@ GEM
     tzinfo (1.2.9)
       thread_safe (~> 0.1)
     unicode-display_width (1.7.0)
-    webpacker (3.5.5)
+    webpacker (4.3.0)
       activesupport (>= 4.2)
       rack-proxy (>= 0.6.1)
       railties (>= 4.2)