Updates clean-css dependency to version 5.

[jakubpawlowicz]

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Version 5 disables rebasing as default so tests changed accordingly.

If you need any help with additional tests or with releasing it, please just let me know! :beer:

[jakubpawlowicz]

Since this is a major version change (incompatible with 4.x) you may have to release it by bumping gulp-clean-css version to 5.0.

[sirwalterious]

@scniro Any progress with this PR? We're facing a bug in clean-css, and would like to update to 5.1.x.

[cb-eli]

Please merge

[iradovanovic]

Please merge this if is a fix. Or this project is dead?

[shrpne]

@scniro friendly reminder, please merge it

[ChaosEngine]

FWIW: You can do this in package.json to enforce usage of clean-css-5.*:

	"overrides": {
		"clean-css": ">=5.3.1"
	}