Depend on 'css' 2.0.0

Causes a couple of source map tests to fail. Does anything look wrong to you @conradz or @lydell? Or is this just a case of updated the expected result to match the new output?

```
  1) rework .toString() sourcemap option should inline sourcemap:

      AssertionError: expected 'body{color:red;}\n/*# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjpudWxsLCJzb3VyY2VzIjpbInNvdXJjZS5jc3MiXSwibmFtZXMiOltdLCJtYXBwaW5ncyI6IkFBQUEsS0FBTyIsInNvdXJjZXNDb250ZW50IjpbImJvZHkgeyBjb2xvcjogcmVkOyB9Il19 */' to be 'body{color:red;}\n/*# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiZ2VuZXJhdGVkLmNzcyIsInNvdXJjZXMiOlsic291cmNlLmNzcyJdLCJuYW1lcyI6W10sIm1hcHBpbmdzIjoiQUFBQSxLQUFPLFNBQVUifQ== */'
      + expected - actual

      +"body{color:red;}\n/*# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiZ2VuZXJhdGVkLmNzcyIsInNvdXJjZXMiOlsic291cmNlLmNzcyJdLCJuYW1lcyI6W10sIm1hcHBpbmdzIjoiQUFBQSxLQUFPLFNBQVUifQ== */"
      -"body{color:red;}\n/*# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjpudWxsLCJzb3VyY2VzIjpbInNvdXJjZS5jc3MiXSwibmFtZXMiOltdLCJtYXBwaW5ncyI6IkFBQUEsS0FBTyIsInNvdXJjZXNDb250ZW50IjpbImJvZHkgeyBjb2xvcjogcmVkOyB9Il19 */"


  2) rework .toString() sourcemapAsObject and sourcemap options should return sourcemap as an object:

      AssertionError: expected 'AAAA,KAAO' to be 'AAAA,KAAO,SAAU'
      + expected - actual

      +"AAAA,KAAO,SAAU"
      -"AAAA,KAAO"
```