Use `grunt-bower-install` for CSS dependencies.

@gojohnnygo made a good point in https://github.com/stephenplusplus/grunt-bower-install/issues/25#issuecomment-30177128 ... the webapp generator ships with JS support for `grunt-bower-install`, but gbi now also handles CSS deps as well.

Thought I'd get a few opinions on this before brute merging it in.

One thought I had was only using gbi for CSS if the user didn't opt into compass. Would that make sense, or not really?