Support for .NET Core Tooling Preview3+ projects

When building API documentation for a project that uses .NET Core Preview 3 or higher (csproj), the build fails, because (roslyn?) is unable to parse PackageReference tags

Functional impact

API documentation can not be built for projects that use .NET Core

Minimal repro steps

1. Create a docFx project
2. Add a metadata reference to a csproj file

Expected result

The build would succeed and API documentation would be built.

Actual result

During the 'generating metadata' phase of the build, a warning is thrown opening the project.
Warning: Error opening project <project>.csproj: The attribute "Version" in element <PackageReference> is unrecognized.. Ignored.

No API documentation is built.

Further technical details

N/A

Sample csproj/docfx.json for reference