Update xUnit 1.9.1 -> 1.9.2

This is fixing the unability of xunit.console.clr4.exe (from xunit.runners package) to run FsCheck tests:
```
xUnit.net console test runner (64-bit .NET 4.0.30319.18408)
Copyright (C) 2013 Outercurve Foundation.
System.IO.FileLoadException: Could not load file or assembly 'xunit, Version=1.9.1.1600, Culture=neutral, PublicKeyToken=8d05b1bb7a6fdb6c' or one of its depende
ncies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
File name: 'xunit, Version=1.9.1.1600, Culture=neutral, PublicKeyToken=8d05b1bb7a6fdb6c'
```