Allow updating vote weight

Currently I'm using `acts_as_votable` as a kind of "`acts_as_ratable`" for one of my models, i.e. all votes are upvotes ranging from 1 to 5 (a.k.a. 5-star ratings).

Currently when a user tries to change her vote (i.e. weight) the vote doesn't register as weight assignment only takes place for a new vote. This pull request simply allows `acts_as_votable` to be seamlessly used as a rating system.