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)
Merge pull request #46 from alphagov/fix-the-crown-for-real-this-time

Really fix the crown