Use gamma 2.40 for rec2020

- [ ] Dart Sass (https://github.com/sass/dart-sass/pull/2729)
- [ ] Tests (https://github.com/sass/sass-spec/pull/2107)
- [ ] Embedded Host (https://github.com/sass/embedded-host-node/pull/418)

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I went through color.js 6 changes, and found this affecting Sass:

- https://github.com/color-js/color.js/pull/672

@nex3 @Goodwine