gradleGen called without required argument 'version'

[heph2]

Building gradle2nix using 'default.nix' fails:

nix-build default.nix

Error:

error: 'gradleGen' at /nix/store/8f5n92zgsrjvhcc9q1ivkjpd1z6wy75k-nixos-21.11.336177.1622abf0e78/nixos/pkgs/development/tools/build-managers/gradle/default.nix:4:15 called without required argument 'version'

       at /tmp/gradle2nix/gradle-env.nix:328:5:

          327|   mkGradle = gradleSpec:
          328|     gradleGen.gradleGen {
             |     ^
          329|       inherit (gradleSpec) nativeVersion;
(use '--show-trace' to show detailed location information)

[lorenzleutgeb]

I believe this could be fixed by merging #49.

/cc @tadfisher