Really fix the crown

* can't just use image-url(...) as a drop-in replacement for url(...)
* Input to image-url has to be relative to the assets path (/assets is a
  no-no)
* Update the sass:check to reflect this (while also fixing the path that
  should have been moved at the same time as the rake tasks were)

In #42 we said we'd fixed this, and we tested it using a shonky manual method (for reasons described therein).  Unfortunately `assets:precompile` locally also generated the unadorned filename (because `spec/dummy` is a Rails 3 app) so we weren't actually testing what we thought we were.