fix font path according to asset pipeline

Because asset pipeline disregards sub-directory name for assets, (e.g. fonts, images .. etc.) less variable for font path should be revised.

With original setup, `@icon-font-path:          "../fonts/";` makes asset path of `/assets/twitter/twitter/fonts/`.
However, the right path is `/assets/twitter/bootstrap/`. So I changed `@icon-font-path` accordingly.