Update CSSLint.js

Hi if I call verify without a ruleset and the fallback case **getRuleset** is called i get:

```
node_modules/csslint/lib/csslint-node.js:6564
            ruleset = this.getRuleset();
                           ^
TypeError: Object #<Object> has no method 'getRuleset'
```
changing this to ```api.getRuleset()``` fixes the problem