Handle ranking before validation

[yaymukund]

This fixes the breaking spec in #68

After I changed to before_validation, a bunch of specs failed. They failed only because they were using update_attribute, which skips validation. I replaced all uses of update_attribute with update_attributes and all tests pass again.

I think it's a little odd that setting the position is coupled with running validations. I'm not sure how to resolve that without doing ugly stuff like overriding save...