Windows build fails on latest official .NET/F# combo

[et1975]

1. Building c:\sources\FsUnit\src\FsUnit.Xunit\FsUnit.Xunit.fsproj failed with exitcode 1.
2. FS0219: c:\sources\FsUnit\src\FsUnit.Xunit\unknown(1,1): The referenced or default base CLI library 'mscorlib' is binary-incompatible with the referenced F# core library 'C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\FSharp.Core\v4.0_4.3.1.0__b03f5f7f11d50a3a\FSharp.Core.dll'. Consider recompiling the library or making an explicit reference to a version of this library that matches the CLI version you are using.
3. FS0229: c:\sources\FsUnit\src\FsUnit.Xunit\unknown(1,1): Error opening binary file 'C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\FSharp.Core\v4.0_4.3.1.0__b03f5f7f11d50a3a\FSharp.Core.dll': The referenced or default base CLI library 'mscorlib' is binary-incompatible with the referenced F# core library 'C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\FSharp.Core\v4.0_4.3.1.0__b03f5f7f11d50a3a\FSharp.Core.dll'. Consider recompiling the library or making an explicit reference to a version of this library that matches the CLI version you are using.

[et1975]

Actually, this is probably me - switching Xunit projects to 4.5.