Temporarily disable digest assets in production

Rails 4's asset pipeline only pre-compiles assets with digest. This
causes two issues:
 - `font-url` helper in Sass is not referencing assets with the
   digest. This may be a manifestation of the same problem worked-
   around in b0f758fc.
 - various plugins appear not to be using the `asset_path` helpers.

Signed-off-by: Alex Coles <alex@alexbcoles.com>