fixes spec

Author: tansengming

test/plan_builder_spec.rb

@@ -203,6 +203,7 @@
  describe 'when none exists on stripe.com' do
  let(:headers) { load_request_fixture('stripe_plans_headers_2017.json') }
  before do
+ Stripe.api_version = '2018-02-04'
  Stripe::Plan.stubs(:retrieve).raises(Stripe::InvalidRequestError.new("not found", "id"))
 
  stub_request(:get, "https://api.stripe.com/v1/plans").