chore(deps): update dependency webpack-merge to v4.2.1

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change	References
webpack-merge	devDependencies	minor	4.1.5 -> 4.2.1	source

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Release Notes

survivejs/webpack-merge

### [`v4.2.1`](https://renovatebot.com/gh/survivejs/webpack-merge/blob/master/CHANGELOG.md#​421--2019-01-04) [Compare Source](https://renovatebot.com/gh/survivejs/webpack-merge/compare/v4.1.5...v4.2.1) ================== - Feature - Support `oneOf` at `merge.smart`. [#​111](https://renovatebot.com/gh/survivejs/webpack-merge/issues/111) - Fix - If there's only single array to merge, clone it. [#​106](https://renovatebot.com/gh/survivejs/webpack-merge/issues/106)

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