Update dependency webpack-dev-server to v3.8.1

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
webpack-dev-server	devDependencies	patch	3.8.0 -> 3.8.1

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Release Notes

webpack/webpack-dev-server

### [`v3.8.1`](https://togithub.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md#​381httpsgithubcomwebpackwebpack-dev-servercomparev380v381-2019-09-16) [Compare Source](https://togithub.com/webpack/webpack-dev-server/compare/v3.8.0...v3.8.1)

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