Also cache NuGet download folder in travis.
Also cache NuGet download folder in travis.

jhamm/FsCheck,paulyoung/FsCheck,ErikSchierboom/FsCheck,fscheck/FsCheck,fscheck/FsCheck,cloudRoutine/FsCheck,cloudRoutine/FsCheck,sideeffffect/FsCheck,ploeh/FsCheck,paulyoung/FsCheck,fsharp/FsCheck