update to serilog 1.2.26 and servicestack 4.0.12

@nickvane,

Here is a pull request for ServiceStack.Logging.Serilog.  As I'm evaluating ServiceStack, I was glad to see that this project existed, but found that it didn't work when I installed the ServiceStack and Serilog packages into my project.

I simply updated ServiceStack.Logging.Serilog to use the latest versions of these packages.

Thanks!