Set `$(VisualStudioVersion)==15.0` in the app.config
Set `$(VisualStudioVersion)==15.0` in the app.config

VS2017's command prompt sets an environment variable of the same name,
with a comment:

```
@REM Set VisualStudioVersion for compatibility with previous revisions of the
@REM VS Developer Command Prompt.
set "VisualStudioVersion=15.0
```

And the IDE seems to have the property set.

This is required because there might be targets that need to use it
before `Microsoft.Common.props`, which imports
`Microsoft.VisualStudioVersion.v*`, is imported. VSMac will be setting
this in it's builder app.config .

This fixes https://bugzilla.xamarin.com/show_bug.cgi?id=54653 by
allowing `Microsoft.Portable.CurrentVersion.props` to be imported.

mono/msbuild,mono/msbuild,mono/msbuild,mono/msbuild