Add a simple spec for validating presence of an _order field.

[yaymukund]

This is the same issue as #51, but I've added a breaking test.

Basically, before_save :handle_ranking gets run only after the validation throws the error. This is because validation callbacks happen first:

[yaymukund]

Unfortunately, I'm not sure how to fix this without breaking a bunch of other specs. Maybe you can defer validations on ranking columns until handle_ranking has been run. I'm not sure.

[mixonic]

Or run the ranking before_validation instead of before_create?

[yaymukund]

I tried exactly that. It breaks a bunch of specs, though I didn't dig in too much.