updates Recurly_Coupon for free trial coupons

[lmnleaf]

Description: Adds attributes for free trial coupons.

Testing the change:
create a free trial coupon by creating a coupon with the following attributes and using "free_trial" for the discount type

$coupon->discount_type = 'free_trial';
$coupon->free_trial_amount = 5;
$coupon->free_trial_unit = "month";

once the coupon is created, get, update, and delete the coupon

[bhelx]

👍