Update SDK to 3.0 Preview 1

[natemcmaster]

Let's update our build to use the latest bits in the 3.0 Preview 1 SDK. The version we're currently using is several weeks stale.

Notable changes:

• Improved framework reference support -- We will likely need to make changes to our WebSdk projects to react to the addition of implicit Microsoft.AspNetCore.App references.
• Updated NuGet and MSBuild versions

cc @NTaylorMullen

[natemcmaster]

Blocked on dotnet/sdk#2867