upgrade dotnet SDK to 2.1.3 on kokoro linux

kokoro/linux/prepare_build_linux_rc

@@ -7,7 +7,7 @@ sudo sh -c 'echo "deb [arch=amd64] https://packages.microsoft.com/repos/microsof
 sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys EB3E94ADBE1229CF
 sudo apt-get update
 # We use the .NET Core SDK 2.x to build...
-sudo apt-get install -y dotnet-sdk-2.0.3
+sudo apt-get install -y dotnet-sdk-2.1.3
 # But we also need the 1.x framework to test against, as we
 # target netstandard1.x
 sudo apt-get install -y dotnet-sharedframework-microsoft.netcore.app-1.0.5