We can download the zip for Unity from https://developer.amazon.com/sdk-download which contains the AmazonMobileAds package. The latest version is 1.0 (May 4, 2016). But this package doesn't work on its own - it also requires the AmazonAds.framework in order to build in Xcode.
The AmazonAds.framework itself can be found in the AmazonMobileAds component of the iOS zip at the same download page. Its version is 2.2.15 (Aug 30, 2016).
Is this recent framework compatible with the older plugin? There seems to be no documentation explaining this.