Use dhallj

[kubukoz]

In #6, we found an issue with dhallj's handling of unions: travisbrown/dhallj#185 not an issue

A workaround is being used (dhall-to-json CLI tool), but ideally we could switch to dhallj at some point.

Here's the issue to track that effort.

We should move to dhallj+circe for handling our Dhall integration. That should make it easier to handle #189, too.

[kubukoz]

Update: this cause is lost, we'll use Nix instead of Dhall. Some work being done in #253.