Update to Less 2.X

Currently plugin works with Less 1.X but in Less 2.X endpoint has changed, so your code should be like:

``` js
less.parse(contents, parseConfig, function(err, out) {
if (err) {
onError(new Error("LESS parse error in " + err.filename + " on line " + err.line + ", column " + err.column + ": " + err.message));
} else {
try {
var css = out.css;
onSuccess(css);
} catch(ex) {
onError(ex);
}
}
});
```

I can do pull request, but I'd prefer if you will fix it yourself :)