Resolve System.Xml.ReaderWriter downgrade error

[stakx]

Installing the newly released .NET Core SDK 2.1 appears to have the side effect of breaking our build:

Castle.Core\src\Castle.Core.Tests\Castle.Core.Tests.csproj : error NU1605:
Detected package downgrade: System.Xml.ReaderWriter from 4.3.1 to 4.3.0. Reference the package directly from the project to select a different version.
 Castle.Core.Tests -> Microsoft.NETCore.App 1.1.8 -> System.Xml.ReaderWriter (>= 4.3.1)
 Castle.Core.Tests -> System.Xml.ReaderWriter (>= 4.3.0)

This PR resolves the downgrade error.

[jonorossi]

Thanks