Authorize.Net `AUD` Accepted

[jaswrks]

We need to add AUD as an accepted currency code in Pro Form generators for Authorize.Net.

[jaswrks]

Next Release Changelog:

• (s2Member Pro) Authorize.Net The AUD currency code is now supported by Authorize.Net, and thus, s2Member Pro has been updated to support the AUD currency code for Pro Forms integrated with Authorize.Net. See this GitHub issue if you'd like further details.