Fix nlohmann_json dependency

Dependencies

• [2] FJ5H26J6 Fix building against nix master
• [3] G7MH6KS7 Add boost dependency
• [4] VPAUFJTI Use PostgreSQL 9.5
• [5] ICYJNJWH Revert "Move build logic for the core hydra package into its own default.nix."
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 135
  [3.16]→[2.115:139](∅→∅)
  − nlohmann_json
  [3.16]

  [3.2651]
  + (nlohmann_json.override { multipleHeaders = true; })