Update Microsoft.NET.Test.Sdk

Author: bording

LibGit2Sharp.Tests/LibGit2Sharp.Tests.csproj

@@ -23,7 +23,7 @@
     <PackageReference Include="xunit.skippablefact" Version="1.3.3" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
     <PackageReference Include="xunit.runner.console" Version="2.2.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.5.0-preview-20170727-01" />
   </ItemGroup>
 
 </Project>