Stripe no longer returns active_card. 

In users model. line 61 we have 
```
self.last_4_digits = customer.active_card.last4
```
Stripe changes their response object: https://stripe.com/docs/api?lang=ruby#retrieve_customer
We no longer have active_card so this will lead to an error.
I update the code to use the new card data with this line
```
self.last_4_digits = customer.cards.data.first.last4
```