Update NETStandard dependencies to stable versions

[emgarten]

json-ld.net currently uses pre-release dependencies for NETStandard. Now that stable versions are available the project should be updated to use the latest stable packages.

https://github.com/NuGet/json-ld.net/blob/353e89eafdba1bee481925ee710d2407a24c641a/src/json-ld.net/project.json#L39-L42

The project should also be updated to csproj instead of xproj.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. After 30 days from now, it will be closed if no further activity occurs. Thank you for your contributions.

[goofballLogic]

Reopening as part of #59

[stale]

This issue has been automatically marked as stale because it has not had recent activity. After 30 days from now, it will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. After 30 days from now, it will be closed if no further activity occurs. Thank you for your contributions.