Bump webpacker from 4.2.2 to 5.1.1

Bumps webpacker from 4.2.2 to 5.1.1.

Changelog

Sourced from webpacker's changelog.

[redacted](https://github.com/rails/webpacker/compare/v5.1.0...v5.1.1) - 2020-04-20

• Update TypeScript documentation and installer to use babel-loader for typescript.[(#2541](https://github.com/rails/webpacker/pull/2541)

[redacted](https://github.com/rails/webpacker/compare/v5.0.1...v5.1.0) - 2020-04-19

• Remove yarn integrity check #2518
• Switch from ts-loader to babel-loader #2449
  Please see the TypeScript documentation to upgrade existing projects to use typescript with 5.1
• Resolve multi-word snakecase WEBPACKER_DEV_SERVER env values #2528

[redacted](https://github.com/rails/webpacker/compare/v5.0.0...v5.0.1) - 2020-03-22

• Upgrade deps and fix sass loader config options bug #2508

[redacted](https://github.com/rails/webpacker/compare/v4.2.2...v5.0.0) - 2020-03-22

• Bump minimum node version #2428
• Bump minimum ruby/rails version #2415
• Add support for multiple files per entry #2476

  entry: {
    home: ['./home.js', './home.scss'],
    account: ['./account.js', './account.scss']
  }

You can now have two entry files with same names inside packs folder, home.scss and home.js

And, other minor fixes, please see a list of changes here

Commits

• 40a1710 5.1.1
• f651362 Added TypeScript upgrade docs for Webpacker 5.1 (#2541)
• d08e9ef Remove references to config/webpack/shared.js in commnets (#2540)
• 559d592 5.1.0
• 4928153 Fix 'environemnt' spelling error in README (#2535)
• 019adcc Update env.md (#2538)
• 7755023 Switch from ts-loader to babel-loader (#2449)
• a84a4bb avoid asset_path route helper collisions (#2511)
• 6f43982 Resolve multi-word snakecase WEBPACKER_DEV_SERVER env values (#2528)
• 1e68429 Update README installation to reflect 5.x version (#2524)
• Additional commits viewable in compare view