Bump dependency versions

[Chris Warburton]

Would be nice to run on newer GHC and Nixpkgs versions.

Using newer cabal versions (2.4) seems to require specifying the
component for 'cabal repl', i.e. we can't do 'cabal repl' we need to do
'cabal repl lib:nix-eval', which is a big naff.

Bumping to Nixpkgs 19.03 gives us GHC 8.6.4, but this package set seems
to fail the test.sh script, due to some test failures of the
microlens-aeson package. It *seems* like these might be too specific,
e.g. saying that '[4, 7]' doesn't match '[7, 4]'; perhaps they meant to
use set equality rather than list equality?