AppVeyor: Install the .NET Core SDK on the fly, and use VS2017 image
AppVeyor: Install the .NET Core SDK on the fly, and use VS2017 image

Note that we don't install the runtime separately, as that's
installed as part of the SDK.

BenJenkinson/nodatime,nodatime/nodatime,malcolmr/nodatime,malcolmr/nodatime,malcolmr/nodatime,nodatime/nodatime,malcolmr/nodatime,jskeet/nodatime,jskeet/nodatime,BenJenkinson/nodatime