We now have Jenkins enabled for `gRPC-haskell` which verifies that
`nix-build release.nix` passes for each pull request.
`master` currently doesn't build, and this is the first pull request to fix the
build by adding the recently introduced `sorted-list` dependency to the `nix`
build. This was done by running `cabal2nix . > default.nix` and also adding a
newer version of `sorted-list` to the `release.nix` file. `nixos-16.03`
uses an older version of `sorted-list` which doesn't have an `IsList` instance for
`SortedList`.
