Update postcss-cssnext to the latest version 🚀

[greenkeeper]

Version 3.1.0 of postcss-cssnext was just published.

Dependency	postcss-cssnext
Current Version	2.11.0
Type	devDependency

The version 3.1.0 is not covered by your current version range.

If you don’t accept this pull request, your project will work just like it did before. However, you might be missing out on a bunch of new features, fixes and/or performance improvements from the dependency update.

It might be worth looking into these changes and trying to get this project onto the latest version of postcss-cssnext.

If you have a solid test suite and good coverage, a passing build is a strong indicator that you can take advantage of these changes directly by merging the proposed change into your project. If the build fails or you don’t have such unconditional trust in your tests, this branch is a great starting point for you to work on the update.

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Release Notes

3.1.0

• Update postcss-font-family-system-ui to version 3.0
  by @JLHwung in #443
• Update postcss-cli usage example in docs
  by @RyanZim in #444

Commits

The new version differs by 35 commits.

• 19fded3 3.1.0
• 9b1bfc2 Fix nesting test
• 0d2cd29 Update deps in yarn.lock
• f96791a Fix travis badge location
• a8f1e79 Update yarn.lock file
• 150ec2f Update postcss-font-family-system-ui to version 3.0 (#443)
• 9109e32 Website: Update postcss-cli usage example (#444)
• ffe3b99 Merge pull request #433 from DanielRuf/master
• 726b0bd fix typo
• 8f092e1 Bye codesponsor 😢
• f3b60a9 Delete package-lock.json
• 1aaaef5 Merge pull request #426 from sethbergman/master
• abf0785 Merge pull request #1 from sethbergman/documentation-fix
• c0353a2 features.md documentation fix
• 7059a7f Update README.md

There are 35 commits in total.

See the full diff

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