Dob validation

I wrote rspec tests when I submitted the pr for dob_validation. I can remember writing them. Somehow, though, they didn't come through to the pr. I've long since deleted the branch so can't see what went wrong. 
Here is a small pr to put the tests back in to the model specs.
What's your opinion of [this guideline from betterspecs](http://betterspecs.org/#describe)? Here, it would be `describe '#orphans_dob_within_1yr_of_fathers_death'`. (Also, semantically, `describe` is a better fit here than `context`, but that's me being pedantic :smile:.)