fix(deps): update dependency postcss-cssnext to v3

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
postcss-cssnext (source)	dependencies	major	^2.11.0 -> ^3.0.0

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Release Notes

MoOx/postcss-cssnext

### [`v3.1.0`](https://togithub.com/MoOx/postcss-cssnext/blob/master/CHANGELOG.md#​310---2018-01-22) [Compare Source](https://togithub.com/MoOx/postcss-cssnext/compare/3.0.2...3.1.0) - Update postcss-font-family-system-ui to version 3.0 by [@​JLHwung](https://togithub.com/JLHwung) in [#​443](https://togithub.com/MoOx/postcss-cssnext/pull/443) - Update postcss-cli usage example in docs by [@​RyanZim](https://togithub.com/RyanZim) in [#​444](https://togithub.com/MoOx/postcss-cssnext/pull/444) ### [`v3.0.2`](https://togithub.com/MoOx/postcss-cssnext/blob/master/CHANGELOG.md#​302---2017-07-10) [Compare Source](https://togithub.com/MoOx/postcss-cssnext/compare/3.0.1...3.0.2) - Fixed: remove incorrect "postinstall" npm script ([#​404](https://togithub.com/MoOx/postcss-cssnext/issues/404) - [@​MoOx](https://togithub.com/MoOx)) ### [`v3.0.1`](https://togithub.com/MoOx/postcss-cssnext/blob/master/CHANGELOG.md#​301---2017-07-10) [Compare Source](https://togithub.com/MoOx/postcss-cssnext/compare/3.0.0...3.0.1) - Fixed: specify the actual require peer dependency for caniuse database (caniuse-lite is used since latest caniuse-api latest major bump) ([@​MoOx](https://togithub.com/MoOx)) - Fixed: bump dependencies not updated to PostCSS@6 ([#​401](https://togithub.com/MoOx/postcss-cssnext/issues/401) - [@​MoOx](https://togithub.com/MoOx)) ### [`v3.0.0`](https://togithub.com/MoOx/postcss-cssnext/blob/master/CHANGELOG.md#​300---2017-07-05) [Compare Source](https://togithub.com/MoOx/postcss-cssnext/compare/2.11.0...3.0.0) #### Removed: support for node 0.12 Node 4+ is supported. #### Changed: upgrade to PostCSS 6 All postcss plugins dependencies have been updated to latest version to ensure full PostCSS 6 compatibility. Some breaking (minor) changes: - image-set polyfill change 2x from 144dpi to 192dpi - whitespace changes for image-set polyfill output - rebeccapurpule is now an hexa number - custom-selector does not output useless (empty) blocks with a selector with no rules associated You can expect some other since all postcss plugins used have been updated to latest versions. If you have unexpected regression, please check corresponding plugins changelog before opening an issue. #### Added: warning is emitted is you use custom property sets and `@apply` This feature won't be included in next the major release of postcss-cssnext. This most likely won't get any more support from browser vendors as the spec is yet considered deprecated and alternative solutions are being discussed. Read more about the reason here

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