Use stable version of Microsoft.NET.Test.Sdk package in Client.Tests project.

Author: tintoy

test/Client.Tests/Client.Tests.csproj

@@ -17,7 +17,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.3.0-preview-20170628-02" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.3.0" />
     <PackageReference Include="xunit" Version="2.3.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.3.0" />