Update clean-css to version 3.4.20 🚀

[greenkeeperio-bot]

Hello lovely humans,

clean-css just published its new version 3.4.20.

State	Update 🚀
Dependency	clean-css
New version	3.4.20
Type	devDependency

This version is not covered by your current version range.

Without accepting this pull request your project will work just like it did before. There might be a bunch of new features, fixes and perf improvements that the maintainers worked on for you though.

I recommend you look into these changes and try to get onto the latest version of clean-css.
Given that you have a decent test suite, a passing build is a strong indicator that you can take advantage of these changes by merging the proposed change into your project. Otherwise this branch is a great starting point for you to work on the update.

Do you have any ideas how I could improve these pull requests? Did I report anything you think isn’t right?
Are you unsure about how things are supposed to work?

There is a collection of frequently asked questions and while I’m just a bot, there is a group of people who are happy to teach me new things. Let them know.

Good luck with your project ✨

You rock!

🌴

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The new version differs by 3 commits .

• a667aae Version 3.4.20.
• c584332 Prevent rules with :selection from being merged (#814)
• 0c6a690 Fix link in readme (#798)

See the full diff.

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This pull request was created by greenkeeper.io.

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