Update clean-css to v2.2.20.
Update clean-css to v2.2.20.

bryanerayner/html-minifier,kangax/html-lint,MacManus/html-minifier,DChristiany/html-minifier,MacManus/html-minifier,GerHobbelt/html-minifier,psychoss/html-minifier,DChristiany/html-minifier,AshKyd/html-minifier,bryanerayner/html-minifier,psychoss/html-minifier,jonnochoo/html-minifier,kangax/html-minifier,jonnochoo/html-minifier,kangax/html-lint,kangax/html-minifier,AshKyd/html-minifier,GerHobbelt/html-minifier