migrate everything to netstandard1.5

src/csharp/Grpc.Auth/project.json

src/csharp/Grpc.Core.Tests/AppDomainUnloadTest.cs

src/csharp/Grpc.Core.Tests/NUnitMain.cs

src/csharp/Grpc.Core.Tests/SanityTest.cs

src/csharp/Grpc.Core.Tests/project.json

src/csharp/Grpc.Core/GrpcEnvironment.cs

src/csharp/Grpc.Core/Internal/NativeExtension.cs

src/csharp/Grpc.Core/Internal/PlatformApis.cs

src/csharp/Grpc.Core/project.json

src/csharp/Grpc.Examples.MathClient/project.json

src/csharp/Grpc.Examples.MathServer/project.json

src/csharp/Grpc.Examples.Tests/NUnitMain.cs

src/csharp/Grpc.Examples.Tests/project.json

src/csharp/Grpc.Examples/project.json

src/csharp/Grpc.HealthCheck.Tests/NUnitMain.cs

src/csharp/Grpc.HealthCheck.Tests/project.json

src/csharp/Grpc.HealthCheck/project.json

src/csharp/Grpc.IntegrationTesting.Client/project.json

src/csharp/Grpc.IntegrationTesting.QpsWorker/project.json

src/csharp/Grpc.IntegrationTesting.Server/project.json