Update test sdk to latest stable version

Author: xt0rted

tests/SendGrid.Tests/SendGrid.Tests.csproj

@@ -16,7 +16,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0-preview-20170106-08" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0-beta5-build1225" />
     <PackageReference Include="xunit" Version="2.2.0-beta5-build3474" />
     <PackageReference Include="System.Diagnostics.TraceSource" Version="4.3.0" />