Update xunit version in dir.props (#687)
Update xunit version in dir.props (#687)

This was missed in the update to xunit 2.1.0 RTM in a86122bd0. Tests
passed on machines that had the old package installed but failed on clean
machines.
sean-gilliam/msbuild,Microsoft/msbuild,rainersigwald/msbuild,nikson/msbuild,AndyGerlicher/msbuild,chipitsine/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,nikson/msbuild,cdmihai/msbuild,mono/msbuild,jeffkl/msbuild,jeffkl/msbuild,chipitsine/msbuild,sean-gilliam/msbuild,mono/msbuild,chipitsine/msbuild,mono/msbuild,cdmihai/msbuild,jeffkl/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,Microsoft/msbuild,nikson/msbuild,chipitsine/msbuild,Microsoft/msbuild,sean-gilliam/msbuild,nikson/msbuild,AndyGerlicher/msbuild,mono/msbuild,rainersigwald/msbuild,Microsoft/msbuild,cdmihai/msbuild,jeffkl/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild