Bump clean-css from 4.1.9 to 4.1.11 (#947)

Bumps [clean-css](https://github.com/jakubpawlowicz/clean-css) from 4.1.9 to 4.1.11.
- [Release notes](https://github.com/jakubpawlowicz/clean-css/releases)
- [Changelog](https://github.com/jakubpawlowicz/clean-css/blob/master/History.md)
- [Commits](clean-css/clean-css@v4.1.9...v4.1.11)

Signed-off-by: dependabot[bot] <support@github.com>

yarn.lock

@@ -1353,8 +1353,9 @@ class-utils@^0.3.5:
     static-extend "^0.1.1"
 
 clean-css@4.1.x:
-  version "4.1.9"
-  resolved "https://registry.yarnpkg.com/clean-css/-/clean-css-4.1.9.tgz#35cee8ae7687a49b98034f70de00c4edd3826301"
+  version "4.1.11"
+  resolved "https://registry.yarnpkg.com/clean-css/-/clean-css-4.1.11.tgz#2ecdf145aba38f54740f26cefd0ff3e03e125d6a"
+  integrity sha1-Ls3xRaujj1R0DybO/Q/z4D4SXWo=
   dependencies:
     source-map "0.5.x"