Bump terser-webpack-plugin from 2.3.0 to 2.3.1

Bumps [terser-webpack-plugin](https://github.com/webpack-contrib/terser-webpack-plugin) from 2.3.0 to 2.3.1.
- [Release notes](https://github.com/webpack-contrib/terser-webpack-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/terser-webpack-plugin/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/terser-webpack-plugin/compare/v2.3.0...v2.3.1

)

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