upgrade Moq to version that supports netstandard1.3

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

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

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

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

...rp/Grpc.IntegrationTesting/Grpc.IntegrationTesting.csproj

src/csharp/Grpc.IntegrationTesting/packages.config

src/csharp/Grpc.IntegrationTesting/project.json

...harp/Grpc.IntegrationTesting.Client/project.json.template

...p/Grpc.IntegrationTesting.QpsWorker/project.json.template

...harp/Grpc.IntegrationTesting.Server/project.json.template

...rpc.IntegrationTesting.StressClient/project.json.template

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