Add validation for credit card owner birthday format

When doing this:

 ``` ruby
credit_card = MyMoip::CreditCard.new(owner_birthday: '80/01/1990')
```

I was getting an `ArgumentError - invalid date` error, but I want to be able to do:

 ``` ruby
credit_card = MyMoip::CreditCard.new(owner_birthday: '80/01/1990')
if credit_card.valid?
  # do something
else
  # do something else
end
```