Use grunt-sass instead of grunt-contrib-sass

`grunt-sass` is a lot faster than `grunt-sass-contrib` that this project is using. It's built on top of a C/C++ implementation of SASS compiler. It lacks support for SASS style, but I don't think we're using that syntax at all in this project, so it should be fine (correct me if I'm wrong!).

run `npm install` then `grunt`

From `grunt-sass` repo:
> This task uses libsass which is an experimental Sass compiler in C++. In contrast to the original Ruby compiler, this one is much faster, but is missing some features, though improving quickly. It also doesn't support Compass. Check out grunt-contrib-sass if you prefer something more stable, but slower.