Update F# Version to most current (6)
Update F# Version to most current (6)

I got interested in this because of #424 -- without F# 5, Expecto can't test any
assemblies that use F# 5 string interpolation. Since I opened that issue, F# 6
has come out. Might as well use that!

Closes #424

haf/expecto,haf/expecto