Appveyor CI doesn't seem to work with win81. Re-enable it.

[ianfixes]

Appveyor CI has a problem when tests are run automatically: win81 tests are "detected", but no xUnit .dlls are generated.

%xunit20%\xunit.console.x86 "C:\projects\manticore-native\runtime\win\Manticore.Win81.Test\obj\x86\Debug\Manticore.Win81.Test.dll" -appveyor
xUnit.net Console Runner (32-bit .NET 4.0.30319.42000)
System.InvalidOperationException: Unknown test framework: could not find xunit.dll (v1) or xunit.execution.*.dll (v2) in C:\projects\manticore-native\runtime\win\Manticore.Win81.Test\obj\x86\Debug

(​The full output can be found here, and includes directory listings: ​https://ci.appveyor.com/project/ifreecarve/manticore-native/build/1.0.6-qceofrlu​ )​

​​Desktop and Net4 tests run without issue, but Win81 fails.

The following links did not help me:
appveyor/ci#201
http://www.appveyor.com/docs/appveyor-yml
http://help.appveyor.com/discussions/problems/2508-xunit-2-tests-discovered-but-runner-not-found