Update dependency postcss-css-variables to v0.11.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change	References
postcss-css-variables	devDependencies	minor	0.9.0 -> 0.11.0	source

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Release Notes

MadLittleMods/postcss-css-variables

### [`v0.11.0`](https://renovatebot.com/gh/MadLittleMods/postcss-css-variables/blob/master/CHANGELOG.md#v0110---2018-10-9) [Compare Source](https://renovatebot.com/gh/MadLittleMods/postcss-css-variables/compare/v0.10.0...v0.11.0) - Fix JS-defined variables using `isImportant`, [#​87](https://renovatebot.com/gh/MadLittleMods/postcss-css-variables/pull/87) ### [`v0.10.0`](https://renovatebot.com/gh/MadLittleMods/postcss-css-variables/blob/master/CHANGELOG.md#v0100---2018-9-25) [Compare Source](https://renovatebot.com/gh/MadLittleMods/postcss-css-variables/compare/v0.9.0...v0.10.0) - Cast `opts.variables` variable values to string - Thank you to [@​shonie](https://renovatebot.com/gh/shonie) for the [contribution](https://renovatebot.com/gh/MadLittleMods/postcss-css-variables/pull/84)

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