[Doppins] Upgrade dependency postcss-css-variables to ^0.6.0

[doppins-bot]

Hi!

A new version was just released of postcss-css-variables, so Doppins has upgraded your project's dependency ranges.

Make sure that it doesn't break anything, and happy merging! :shipit:

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Upgraded postcss-css-variables from ^0.5.1 to ^0.6.0

Changelog:

Version 0.6.0

v0.6.0 Changelog (https://github.com/MadLittleMods/postcss-css-variables/blob/master/CHANGELOG.md#v060---2016-9-23)