Fix Webpack build

[dreikanter]

webpacker:compile hangs after start.

Could be caused by erb-loader.

Update: ./bin/webpack --progress --verbose displays this:

ERROR in ./app/javascript/packs/hello_erb.js.erb
Module build failed (from ./node_modules/rails-erb-loader/index.js):
Error: rails-erb-loader failed with code: 1
    at ChildProcess.<anonymous> (/app/node_modules/rails-erb-loader/index.js:128:16)
    at ChildProcess.emit (events.js:200:13)
    at maybeClose (internal/child_process.js:1021:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:283:5)

[dreikanter]

Got this error after clean webpacker installation:

Integrity check: Flags don't match

Fix:

• Rails erb loader Compile Error rails/webpacker#1529
• Error: rails-erb-loader failed with code: 1 usabilityhub/rails-erb-loader#63

Also got this warning:

WARNING: We noticed you're using the useBuiltIns option without declaring a core-js version. Currently, we assume version 2.x when no version is passed. Since this default version will likely change in future versions of Babel, we recommend explicitly setting the core-js version you are using via the corejs option.

You should also be sure that the version you pass to the corejs option matches the version specified in your package.json's dependencies section. If it doesn't, you need to run one of the following commands:

npm install --save core-js@2 npm install --save core-js@3
yarn add core-js@2 yarn add core-js@3

Fix:

yarn add core-js@2