all-packages.nix: comment-out gist, gitlab, gitlab-shell, gem-nix, and...

all-packages.nix: comment-out gist, gitlab, gitlab-shell, gem-nix, and loadRubyEnv to fix evaluation

Nixpkgs evaluation was broken in these attributes by the changes from
https://github.com/NixOS/nixpkgs/pull/4677, I guess.