Remove xunit from cibuild.ps1

I had copied this code from GHfW where we had some timeout issues with the unit tests. That's not a problem with Octokit.

By running our FullBuild, we still get test coverage with no need to duplicate xunit code here.
Merge pull request #150 from octokit/cleanu-up-cibuild

Remove xunit from cibuild.ps1