Fix json_to_record error reporting

I'm not sure how or why I ever typed the addition of an error
handler instead of error_msg here.  One of life's great mysteries.
Update stripe test API key

The old API key suddenly became invalid?  One of life's great mysteries.
Remove customer.subscription (Stripe API changed)

As of Stripe API 2014-01-31, customer.subscription no longer exists
because it's now replaced by a subscriptions object enumerating
the multiple subscriptions a customer may have.

Don't have time right now to add multi-subscription support, so
this commit is removing the old single-subscription member of
a customer.
Bump to version 0.6.7

All tests pass as of this commit on this date.