Rename rating.update to rating.set

In order to have a coherent .update methods on all the resources,
and to have it accept a hash of options, Rating#update is renamed
to Rating#set since it does not accept a hash, but instead it accepts
the exact value to set the rating to.
Merge pull request #36 from Fullscreen/rename-rating-update-to-set

Rename rating.update to rating.set