Fix typo on sass task.

With grunt-sass there is [no option value `style`](https://github.com/sindresorhus/grunt-sass#api) the correct one is `outputStyle`.
So sass files are not being preprocessed as compressed.

I think the confusion came from the grunt-contrib-sass which [uses `style`](https://github.com/gruntjs/grunt-contrib-sass#style) for the same option value.