Added a cabal.config file; the result of running cabal freeze.
It's not used yet (needs a newer cabal than is in debian stable),
but the plan is that once the autbuilders are swiched to jessie,
this can be used to make cabal install the same versions of packages
that this patch got building, and so avoid breaking every time eg, yesod is
upgraded.
This commit was sponsored by Daniel Atlas.
