build: update devdependency webpack-merge to ~4.2.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
webpack-merge	devDependencies	minor	~4.1.1 -> ~4.2.0

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Release Notes

survivejs/webpack-merge

### [`v4.2.2`](https://togithub.com/survivejs/webpack-merge/blob/master/CHANGELOG.md#​422--2019-08-27) [Compare Source](https://togithub.com/survivejs/webpack-merge/compare/v4.2.1...v4.2.2) - Fix - Update minimum version of lodash to 4.17.15 in order to avoid `npm audit warning`. [#​116](https://togithub.com/survivejs/webpack-merge/issues/116) - Docs - Improve `merge.unique` documentation. [#​103](https://togithub.com/survivejs/webpack-merge/issues/103) - Docs - Add clear note about precedence. [#​115](https://togithub.com/survivejs/webpack-merge/issues/115) ### [`v4.2.1`](https://togithub.com/survivejs/webpack-merge/blob/master/CHANGELOG.md#​421--2019-01-04) [Compare Source](https://togithub.com/survivejs/webpack-merge/compare/v4.1.5...v4.2.1) - Feature - Support `oneOf` at `merge.smart`. [#​111](https://togithub.com/survivejs/webpack-merge/issues/111) - Fix - If there's only single array to merge, clone it. [#​106](https://togithub.com/survivejs/webpack-merge/issues/106)

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