Version 2 of our API was retired on April 29, 2016.
The latest version of the API (v2.2) is in beta and offers substantial improvements in performance, functionality and ease of use, so we encourage you to check it out.
While we strongly recommend updating to v2.2, the easiest option to keep your integration up and running is to migrate to v2.1. The migration should be almost seamless as v2 and v2.1 are operationally very similar.
We're here to help you in the process so please do not hesitate to join us in our support community should you have any questions.