An updated version of mashup SDK might not be available for a couple of weeks. Preparing for PegaWorld 2017 has us all quite busy. In the meantime, you can download the previous versions of XCode from Apple here: https://developer.apple.com/download/more/. Apple supports more than one version of XCode coexisting. See the Apple developer forums for more information.
Hopefully, this gets you what you need, and we will have the new version of MashupSDK support 8.3.1 in June.