Compile all the CSS files

so the `.css` files match the `.less` files. This was not done for #150 (since we did not use grunt yet) and caused quite a scare but `grunt diff` shows no differences for any of the books:

Books tested:
- precalculus: 0 diffs
- sociology: 0
- psychology: 0
- economics: 0
- statistics: 0
- anatomy: 0
- biology: 0
- physics: 0