cabal: disallow unordered-containers < 0.2.5.0

Earlier versions of unordered-containers had a faulty
Show instance that made the doctests fail
(since 37ec59e4d5573677ba6a30550d7a8808de37def3).

The Show instance was fixed here:
https://github.com/tibbe/unordered-containers/commit/d7b09ed10b40a4734eba27ae8c812e8114e074b7