added validation to check orphans dob within gestation period of fathers death

The longest and 2nd longest gestation periods are 375 days and 317 days respectively
http://www.newhealthguide.org/Longest-Pregnancy.html

Validation ensures gestation within 317 days.
@sampritipanda is there a more elegant way to fix this?

it seems because this validation runs regardles of other validation I do need it.
When I remove it rspec give me 24 errors similar to the one below:-

 20) Orphan should not allow sponsored_by_another_org to be set to nil
     Failure/Error: it { is_expected.to_not allow_value(nil).for(:sponsored_by_another_org) }
     NoMethodError:
       undefined method `+' for nil:NilClass
     # ./app/models/orphan.rb:24:in `orphans_dob_within_gestation_of_fathers_death'