Fixes tests for Stripe 5.19.0

[gaffneyc]

As of 5.19.0, the compute_signature method was made public and they
split the timestamp into a separate param (which must be a Time). This
version checks compute_signature based on the number of params it takes
since doing a version check felt odd. Now that it is public, the method
shouldn't change it's API.

The build is currently failing with errors like these:

  8) StripeEvent::WebhookController with multiple signing secrets succeeds with valid signature from first secret
     Failure/Error: signature = Stripe::Webhook::Signature.send(:compute_signature, "#{timestamp}.#{payload}", secret)

     ArgumentError:
       wrong number of arguments (given 2, expected 3)
     # ./spec/controllers/webhook_controller_spec.rb:16:in `generate_signature'
     # ./spec/controllers/webhook_controller_spec.rb:28:in `webhook_with_signature'
     # ./spec/controllers/webhook_controller_spec.rb:117:in `block (3 levels) in <top (required)>'

1: stripe/stripe-ruby#915

[coveralls]

Coverage decreased (-0.9%) to 99.099% when pulling 3aea3a0 on gaffneyc:stripe-5-19-0 into 31d759d on integrallis:master.

[rmm5t]

Thank you!