"Mocking external dependencies is actually considered to be a mocking no-no, because then you create a binding between your app and the external dependency's API. Instead, write a thin wrapper around that dependency that speaks in the domain of your app, and mock that as a means of isolating your app's code/specs from that dependency.
blog-post-in-memory-fake-adapters
blog-post-how-to-fake-external-services-in-specs