add remaining_billing_cycles to the writable attributes for subscriptions

[mbeale]

currently does not exist in client lib

[drewish]

@theTrip73 were you running into this as well?

[trip-somers]

No. The note in my code was a reference to trying to figure out how to handle finite subscriptions since Authorize.net's export contains an end date instead of a number of billing cycles remaining.

[bhelx]

It looks like this exists now. Please re-open if you think i'm wrong.

recurly-client-php/lib/recurly/subscription.php

Line 13 in fd21220

'cost_in_cents', 'remaining_billing_cycles', 'bulk', 'terms_and_conditions', 'customer_notes',