Tweaked documentation per what @mostly-meta said

This is about the Javascript block per his comment in #86
Merge branch 'master' of github.com:pydanny/dj-stripe
Customer.default_card replaces .active_card

Customer.active_card was apparently deprecated by Stripe. As a
nearly drop-in replacement, Customer.default_card contains very
comparable values, if the expand=['default_card'] kwarg is passed
to stripe.Customer.retrieve(). This commit updates the Customer
model for this change.