You should update your plugin to .net 4.5 - we've updated due to needing to reference other libraries that are 4.5 only. We still support VS2010, which ships with 4.0, but we expect those users to also include .net 4.5 (4.0 is out of support, after all). Once you've updated to 4.5, you'll need to reinstall the packages you've got, and then you should be able to update to the latest SDK. See the
NuGet docs for more details on reinstalling.
Regards
Matt