Merge/Dependencies.hs: fix broken 'cabalDependency' guesser

Before the fix 'cabalDependency' returned exact ~version of
cabal shipped with GHC, while it was intended to return
>=version.

Our reimplementation of simplifier exposed that bug.

Reported-by: Alexander Vershilov <alexander.vershilov@gmail.com>
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>