Motivation for this change

During the recent refactoring of SPL, the version attribute was mistakenly removed, causing SPL to inherit a version attribute from somewhere in the nixpkgs lib. This version is included in the derivation name, and since it differs between Hydra (18.09pre-git) and normal NixOS machines (something like 18.09pre149424.ce522e4ad27), the SPL is not substituted from the binary cache. Also it should be fixed because this version has nothing to do with SPL.

Things done

This PR adds a correct version attribute to the derivation.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.