Look for F# 3.1 version of F# Interactive

It would also be great to include F# 3.1 `fsi.exe` in the release (unless it depends on the newer version of .NET?). 

I use `executeFSI`  when generating documentation for Deedle and that needs F# 3.1 (the documentation is using new slicing syntax) and so using old FSI does not generate the right tooltips. But that's not a big deal - I added a command to delete the FSI that comes with FAKE and so the one on the current system will be used (I know I can modify the `config` in the package, but I did not want to put that in the Git repo).