Csslint fixes

It looks like the behaviour of `csslint` has changed since the `ArcanistCSSLintLinter` was written. Consequently, `ArcanistCSSLintLinter` does not work with the latest version of `csslint` (v0.10.0).

Whilst the proposed changes work for v0.10.0, it may be more challenging to ensure that these changes don't break with older versions of `csslint`.