chore(deps): bump clean-css from 4.1.9 to 4.2.1 in /dev

[dependabot[bot]]

Bumps clean-css from 4.1.9 to 4.2.1.

Changelog

Sourced from clean-css's changelog.

4.2.1 / 2018-08-07

• Fixes giving breakWith option via a string.

4.2.0 / 2018-08-02

• Adds process method for compatibility with optimize-css-assets-webpack-plugin.
• Fixed issue #861 - new transition property optimizer.
• Fixed issue #895 - ignoring specific styles.
• Fixed issue #947 - selector based filtering.
• Fixed issue #964 - adds configurable line breaks.
• Fixed issue #986 - level 2 optimizations and CSS 4 colors.
• Fixed issue #1000 - carriage return handling in tokenizer.
• Fixed issue #1038 - font-variation-settings quoting.
• Fixes ReDOS vulnerabilities in validator code.

4.1.11 / 2018-03-06

• Backports fixes to ReDOS vulnerabilities in validator code.

4.1.10 / 2018-03-05

• Fixed issue #988 - edge case in dropping default animation-duration.
• Fixed issue #989 - edge case in removing unused at rules.
• Fixed issue #1001 - corrupted tokenizer state.
• Fixed issue #1006 - edge case in handling invalid source maps.
• Fixed issue #1008 - edge case in breaking up font shorthand.

Commits

• 87a8722 Version 4.2.1.
• b7da0d1 Fixes giving breakWith option via a string.
• e14931a Bumps web interface to version 4.2.0.
• 9dd091f Version 4.2.0.
• 59bf990 Fixes #964 - allows for configurable line breaks.
• 48808f7 Fixes #1000 - carriage return handling in tokenizer.
• ec259be Fixes #986 - CSS 4 colors in level 2 optimizations.
• 8f52600 Fixes #1036 - double deepClone declaration.
• 5603448 Updates changelog - see #1037.
• 88c558a Fix incorrect stripping of quotes in font-variation-settings (#1039)
• Additional commits viewable in compare view

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