Fix woff mimetype to be consistent with latest W3 spec.

The W3 spec now says that mimetype for woff fonts should be: application/font-woff

http://www.w3.org/TR/WOFF/#appendix-b

See:
  * http://zduck.com/2013/google-chrome-and-woff-font-mime-type-warnings/
  * http://stackoverflow.com/questions/16704967/resource-interpreted-as-font-but-transferred-with-mime-type-application-x-font-w

note: This was changed before in compass about a year and a half ago: https://github.com/chriseppstein/compass/pull/755  

Now it seems to need changed again.