Prevent build failure with cabal
Prevent build failure with cabal

path-0.5.13, released in March 2017, [adds function
reldir](https://hackage.haskell.org/package/path-0.5.13/changelog).

Local variable reldir shadows it:
https://travis-ci.org/commercialhaskell/stack/jobs/234552558

So the combination doesn't build since March. Sorry this is the quickest fix
possible, I know a few alternatives, but I'm short on time and the choice is a
matter of taste.

anton-dessiatov/stack,MichielDerhaeg/stack,mrkkrp/stack,MichielDerhaeg/stack,anton-dessiatov/stack,martin-kolinek/stack,martin-kolinek/stack,mrkkrp/stack,juhp/stack,juhp/stack