Config file is expected to be at root/.assets.rb

It can still be set to a different path, but Pannierfile was a silly filename.
This is better, because:

  * It has a .rb extension, which is more descriptive and will be picked up by
    syntax highlighters
  * It's less self-centred on the part of the Pannier project
  * It starts with a dot, so it will always be near the top in a sorted
    directory