loggly update to 3.5.1

Hi there - I'm the new maintainer of the [loggly nuget package](https://www.nuget.org/packages/loggly-csharp) ([git source](https://github.com/neutmute/loggly-csharp)). I've made significant improvements in 3.5 and this PR updates your serilog sink to that version. Adding serilog support gave some useful improvements to the loggly API that I'd improved for the [NLog target](https://github.com/joefitzgerald/nlog-targets-loggly).

I've tested the sink in a quick test project - let me know if you would like this integration test (console app) in your serilog solution. Happy to add it but wasn't sure if you wanted the noise of this extra project.

I hope you find this useful, let me know of any tweaks you require.
cheers
Neutmute