Use Nix 2.0 and default GCC

Dependencies

• [2] ICYJNJWH Revert "Move build logic for the core hydra package into its own default.nix."
• [3] HPIJ35SG
• [4] VPAUFJTI Use PostgreSQL 9.5
• [5] 6K5PBUUN Use buildEnv to combine Hydra's Perl dependencies
• [6] GGWH5GY7 Remove unused dependency on aws-sdk-cpp
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 44
  [2.706]→[2.706:731](∅→∅)
  − nix = nixUnstable;
  [3.1188]

  [2.731]
  + #nix = nixUnstable;
• edit in release.nix at line 105
  [2.2283]→[2.2283:2323](∅→∅)
  −

  − stdenv = overrideCC stdenv gcc6;