marked UpdateServiceFixture as Windows only test

NzbDrone.Core.Test/UpdateTests/UpdateServiceFixture.cs

@@ -30,6 +30,8 @@ namespace NzbDrone.Core.Test.UpdateTests
         [SetUp]
         public void Setup()
         {
+            WindowsOnly();
+
             Mocker.GetMock<IAppFolderInfo>().SetupGet(c => c.TempFolder).Returns(TempFolder);
             Mocker.GetMock<ICheckUpdateService>().Setup(c => c.AvailableUpdate()).Returns(_updatePackage);