NugetRestore not working with Nuget 4 and NuGetMSBuildVersion.MSBuild15

I have `$NUGET_EXE` pointing to nuget v4.1.0.

```
NuGetRestore("sln.sln", new NuGetRestoreSettings {
MSBuildVersion = NuGetMSBuildVersion.MSBuild15
});
```

outputs

```
Cannot find the specified version of msbuild: '15'
An error occurred when executing task 'Restore'.
```

There is related [SO answer](http://stackoverflow.com/questions/43128897/cake-nugetrestore-always-wants-msbuild14#comment73403182_43129179).

Start with the NuGetRestore call and its NuGetRestoreSettings.MSBuildVersion option, using the reported NuGet 4.1.0 and NuGetMSBuildVersion.MSBuild15 reproduction. Trace how the MSBuild version is resolved and check existing restore tests or entry points. Done means the reported restore completes without the "Cannot find the specified version" error.

Written by the indexing model from the issue text.