Merge pull request #74 from AArnott/xunitRunnerUpdate

Update xunit test runner to 2.3.1

Author: AArnott

src/CodeGeneration.Roslyn.Tests/CodeGeneration.Roslyn.Tests.csproj

@@ -15,8 +15,8 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.CSharp" Version="4.4.1" />
-    <PackageReference Include="Xunit" Version="2.2.0" />
-    <PackageReference Include="Xunit.runner.visualstudio" Version="2.2.0" />
+    <PackageReference Include="Xunit" Version="2.3.1" />
+    <PackageReference Include="Xunit.runner.visualstudio" Version="2.3.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
   </ItemGroup>