git push herokuでinvalid configuration objectエラーが出る

前提・実現したいこと

Railsでwebサービスの開発を行っています。

git push herokuをすると、以下のようなエラーが出ます。

remote:        Compiling…
remote:        Compilation failed:
remote:        Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
remote:         - configuration has an unknown property 'optimization'. These properties are valid:
remote:           object { amd?, bail?, cache?, context?, dependencies?, devServer?, devtool?, entry, externals?, loader?, module?, name?, node?, output?, performance?, plugins?, profile?, recordsInputPath?, recordsOutputPath?, recordsPath?, resolve?, resolveLoader?, stats?, target?, watch?, watchOptions? }
remote:           For typos: please correct them.
remote:           For loader options: webpack 2 no longer allows custom properties in configuration.
remote:             Loaders should be updated to allow passing options via loader options in module.rules.
remote:             Until loaders are updated one can use the LoaderOptionsPlugin to pass these options to the loader:
remote:             plugins: [
remote:               new webpack.LoaderOptionsPlugin({
remote:                 // test: /.xxx$/, // may apply this only for some modules
remote:                 options: {
remote:                   optimization: ...
remote:                 }
remote:               })
remote:             ]
remote:         - configuration.module.rules[1] has an unknown property 'sideEffects'. These properties are valid:
remote:           object { enforce?, exclude?, include?, issuer?, loader?, loaders?, oneOf?, options?, parser?, query?, resource?, resourceQuery?, compiler?, rules?, test?, use? }
remote:         - configuration.module.rules[2] has an unknown property 'sideEffects'. These properties are valid:
remote:           object { enforce?, exclude?, include?, issuer?, loader?, loaders?, oneOf?, options?, parser?, query?, resource?, resourceQuery?, compiler?, rules?, test?, use? }
remote:         - configuration.module.rules[3] has an unknown property 'sideEffects'. These properties are valid:
remote:           object { enforce?, exclude?, include?, issuer?, loader?, loaders?, oneOf?, options?, parser?, query?, resource?, resourceQuery?, compiler?, rules?, test?, use? }
remote:         - configuration.module.rules[4] has an unknown property 'sideEffects'. These properties are valid:
remote:           object { enforce?, exclude?, include?, issuer?, loader?, loaders?, oneOf?, options?, parser?, query?, resource?, resourceQuery?, compiler?, rules?, test?, use? }