Bump terser-webpack-plugin from 1.4.1 to 2.2.1 (linuswillner#387)

Bumps [terser-webpack-plugin](https://github.com/webpack-contrib/terser-webpack-plugin) from 1.4.1 to 2.2.1.
- [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](webpack-contrib/terser-webpack-plugin@v1.4.1...v2.2.1)

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