Fix "Unmet peer dependencies" warning

- Showed when run "yarn install --check-files" and push to heroku.
- By run "yarn upgrade webpack@^4.0.0" to add "webpack": "^4.0.0" to 
package.json.
- Followed by run "yarn install --check-files" to update the yarn.lock 
file.
(Source: rails/webpacker#1078)

Author: lethang7794

package.json

@@ -8,7 +8,8 @@
     "@rails/webpacker": "4.2.2",
     "bootstrap": "3.4.1",
     "jquery": "3.5.1",
-    "turbolinks": "^5.2.0"
+    "turbolinks": "^5.2.0",
+    "webpack": "^4.0.0"
   },
   "version": "0.1.0",
   "devDependencies": {

yarn.lock

@@ -7352,7 +7352,7 @@ webpack-sources@^1.0.0, webpack-sources@^1.0.1, webpack-sources@^1.1.0, webpack-
     source-list-map "^2.0.0"
     source-map "~0.6.1"
 
-webpack@^4.41.2:
+webpack@^4.0.0, webpack@^4.41.2:
   version "4.43.0"
   resolved "https://registry.yarnpkg.com/webpack/-/webpack-4.43.0.tgz#c48547b11d563224c561dad1172c8aa0b8a678e6"
   integrity sha512-GW1LjnPipFW2Y78OOab8NJlCflB7EFskMih2AHdvjbpKMeDJqEgSx24cXXXiPS65+WSwVyxtDsJH6jGX2czy+g==