Bump System.Net.Http and Octokit builds to pick up type conflict fix

Fixes #503

When running System.Net.Http in Unity with the 4.6 experimental runtime,
Unity will load Mono's version of System.Net.Http, which causes type conflicts
all over the place given that both dlls are running side by side.
Easiest way of avoiding that is to move all the classes in System.Net.Http
into its own namespace - "DotNetHttp35".
Also renamed the dll to reflect the namespace change.

See https://github.com/github-for-unity/dotnet-httpclient35/commit/8566426a9217