Update tests for new Braintree API

These new URLs were introduced in Braintree 2.32.0:

https://github.com/braintree/braintree_ruby/commit/c2575028d1f2f2cd2b025f56b0112acb3697e5dc#diff-9

This PR would also require a higher version of Braintree...but I assume people using an old version of the gem are seeing breakages anyway.
Surprisingly, no - 32 > 5. You can see the progression here: https://rubygems.org/gems/braintree/versions