asp.net core dependency on System.Xml.Linq

Hi,

The Hangfire packages have a dependency on System.Xml.Linq

however, this package is not supported on .net core


And because this dll is not available, I get an error on the xUnit.net testrunner:

```------ Discover test started ------
[xUnit.net 00:00:00.8776462] Discovering: tests
[xUnit.net 00:00:01.0764595] Discovered: tests
An exception occurred while test discoverer 'VsTestRunner' was loading tests. Exception: Could not load file or assembly 'System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'. The system cannot find the file specified.
========== Discover test finished: 15 found (0:00:01,6077412) ==========
```

If I add this to my my csproj file
```

```

it will fail
```C:\Program Files\dotnet\sdk\1.0.1\NuGet.targets(97,5): error : Package System.Xml.Linq 3.5.21022.801 is not compatible with netcoreapp1.1 (.NETCoreApp,Version=v1.1). Package System.Xml.Linq 3.5.21022.801 supports: net20 (.NETFramework,Version=v2.0)```

Start by inspecting the Hangfire package dependency declarations and reproduce the reported failure with a netcoreapp1.1 project using the xUnit test runner. Check whether the System.Xml.Linq dependency is compatible with .NET Core; done means the packages install and test discovery completes without the missing-assembly error.

Written by the indexing model from the issue text.