Fix incorrect PayPal payment method attribute

The payment method attribute sent to the API should be underscored,
not camel-cased. This fixes the PayPal integration path.

Signed-off-by: Stephen Celis <stephen@recurly.com>
Signed-off-by: Evan Owen <kainosnoema@gmail.com>
Bump version and update changelog

Signed-off-by: Stephen Celis <stephen@recurly.com>
Signed-off-by: Evan Owen <kainosnoema@gmail.com>