Allow doctest 0.25

[wolfgangwalther]

After #562 it built fine briefly, but I think Stackage Nightly then updated doctest. Builds fine for me in Nixpkgs with jailbreaking. Changelog looks unsuspicious: https://hackage.haskell.org/package/doctest-0.25.0/changelog

[Shimuuar]

I've updated cabal metadata for 0.13.2.0