update to version 4 of IUCN Red List API

[jeffreyhanson]

The aoh package depends on the rredlist package to handle IUCN Red List API queries (https://docs.ropensci.org/rredlist/). Currently, the rredlist package is only compatible with version 3 of the IUCN Red List API. Since version 3 of the IUCN Red List API is being deprecated, it is no longer possible to get API keys for version 3 of the API (but see here to get a key, ropensci/rredlist#52 (comment)) and soon version 3 of the API will stop functioning entirely (https://apiv3.iucnredlist.org/). As such, the aoh package will need to be updated to maintain compatibility with the IUCN Red List API. My plan is to wait until the rredlist package is updated to be compatible with version of 4 the API (ropensci/rredlist#52), and then update the aoh package to maintain compatbility with the rredlist package. In the meantime, if you're having trouble using this package due to issues with the IUCN Red List API, please post about it here and I'll see what I can do.