Reference MicroBuild.Core unconditionally

The conditional reference caused this package to not apply to the .NET Core build, meaning that the NuGet package was only partially signed, not including the netcoreapp2.0 one.

Fixes microsoft#71.

Co-authored-by: Ben Villalobos <bevillal@microsoft.com>

Merge pull request microsoft#37 from Microsoft/master

FI master to release/1.0

Drop commit ID from release branches (microsoft#27)

Proactively check for loaded MSBuild (microsoft#14)

The .NET assembly loader ensures that any assembly referenced by a
method is loaded before the method is called. This requires that
MSBuildLocator be called **before any method that references MSBuild**.

That's a confusing requirement, and easy to forget. But it's easy to
check for, and since we expect the locator to be called roughly once per
process, not too expensive to check for at `RegisterInstance` time.

Produces an error like

```
Unhandled Exception: System.InvalidOperationException: Microsoft.Build.Locator.MSBuildLocator.RegisterInstance was called, but MSBuild assemblies were already loaded.
Ensure that RegisterInstance is called before any method that directly references types in the Microsoft.Build namespace has been called.
Loaded MSBuild assemblies: Microsoft.Build, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
   at Microsoft.Build.Locator.MSBuildLocator.RegisterInstance(VisualStudioInstance instance)
   at Microsoft.Build.Locator.MSBuildLocator.RegisterDefaults()
   at MultiProcBuilderApplication.Program.Main(String[] args) in s:\work\MultiProcBuilderApplication\MultiProcBuilderApplication\Program.cs:line 13
```

Update version.json