Move to stable xUnit MSBuild runner (v2.3.1)

- relates to #65
- follow-up to 634c9d2

Author: dougbu

.nuget/packages.config

@@ -3,5 +3,5 @@
   <package id="Microsoft.Web.SkipStrongNames" version="1.0.0" />
   <package id="Microsoft.Web.StyleCop" version="1.0.0" />
   <package id="StyleCop" version="5.0.0" />
-  <package id="xunit.runner.msbuild" version="2.3.1-rc2-build3844" targetFramework="net452" />
+  <package id="xunit.runner.msbuild" version="2.3.1" targetFramework="net452" />
 </packages>