Upgrade xunit to version 2.4.1-pre.build.4071 (#3)

They're taking a long time to release 2.4.1 and it's only a few commits,
we need one of them.

Also use the NS1.x binaries instead so 2018-xx branches which don't build
netstandard.dll for monodroid etc can use it.

In fact, xunit removed the NS2.0 binaries again in 2.4.1 so we don't even have much choice :)

README.md

@@ -1,5 +1,5 @@
 # xunit-binaries
 
-Extracted xUnit nuget packages targetting netstandard 2.0 (or 1.1 when not available)
+Extracted xUnit nuget packages targetting netstandard 1.x
 
 Xunit.NetCore.Extensions.dll were extracted from https://dotnet.myget.org/feed/dotnet-buildtools/package/nuget/Microsoft.xunit.netcore.extensions/2.1.0-rc1-03006-01