Update System.Net.Http references

[Blackbaud-JeffDye]

Addresses #77
Based off of the changes in datalust/serilog-sinks-seq#85

I haven't modified references like this before so please check over all this thoroughly, there might be something that I've missed or failed to account for.

Also, I needed to update xunit and xunit.runner.visualstudio to 2.3.1 in order to run tests locally, not sure if that's due to using latest Visual Studio and/or .Net 4.7.1 - I've held off on pushing that commit, but I can if it isn't an issue for y'all.

Thanks!

[Blackbaud-JeffDye]

Really sorry to bother you while you're out on holiday @merbla, if it isn't too much trouble to check this out whenever you have a second? Perhaps @nblumhardt could give these changes a quick sanity check as he's the author of the changes in serilog-sinks-seq that I based these off of?

Thanks guys!

[nblumhardt]

@Blackbaud-JeffDye thanks for the PR! @merbla I think since this is going into dev it's a good opportunity to vet the changes - LGTM, I'll hit the button and beg forgiveness if this all goes horribly wrong ;-)