Add upper bound on `libBF` dependency

[yav]

This should help avoid build failures if we make backward incompatible changes to libBF

[robdockins]

This has already been done.

cryptol/cryptol.cabal

Line 60 in 5f3ff5b

libBF >= 0.6 && < 0.7,

However, we should edit the upper bounds in Hackage.

[robdockins]

@atomb, looks like you are the package maintainer on Hackage. Would you be able to add the libBF < 0.6 constraint for Cryptol 2.10.0?

[robdockins]

Newest hackage release now has an appropriate upper bound.