Fixed sass to work with upstream version of wyrm.

In wyrm v.0.0.84, the sass file `wyrm_core/font_awesome.sass` was removed and a new file `wyrm_core/font_awesome_extra.sass` was included (see  https://github.com/snide/wyrm/commit/d500db57d328661e0b4f4712f4c7317494b6a6c2). This theme still refers to `wyrm_core/font_awesome`, so it won't build if you freshly checkout the repo and try to build the theme.