ruby: Fix evaluation error

In the tarball job:

````
checking find-tarballs.nix
error: while evaluating anonymous function at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/maintainers/scripts/find-tarballs.nix:6:1, called from undefined position:
while evaluating ‘operator’ at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/maintainers/scripts/find-tarballs.nix:27:16, called from undefined position:
while evaluating ‘immediateDependenciesOf’ at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/maintainers/scripts/find-tarballs.nix:39:29, called from /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/maintainers/scripts/find-tarballs.nix:27:44:
while evaluating anonymous function at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/lib/attrsets.nix:224:10, called from undefined position:
while evaluating anonymous function at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/maintainers/scripts/find-tarballs.nix:40:37, called from /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/lib/attrsets.nix:224:16:
while evaluating ‘derivationsIn’ at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/maintainers/scripts/find-tarballs.nix:42:19, called from /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/maintainers/scripts/find-tarballs.nix:40:40:
while evaluating ‘optional’ at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/lib/lists.nix:175:20, called from /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/maintainers/scripts/find-tarballs.nix:44:33:
while evaluating ‘canEval’ at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/maintainers/scripts/find-tarballs.nix:48:13, called from /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/maintainers/scripts/find-tarballs.nix:44:43:
while evaluating the attribute ‘pkgs’ of the derivation ‘ruby-dev-2.3.1-p0’ at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/pkgs/build-support/trivial-builders.nix:10:14:
while evaluating ‘override’ at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/lib/customisation.nix:60:22, called from /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/pkgs/development/interpreters/ruby/dev.nix:10:13:
while evaluating ‘makeOverridable’ at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/lib/customisation.nix:54:24, called from /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/lib/customisation.nix:60:31:
anonymous function at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/pkgs/development/ruby-modules/bundix/default.nix:1:1 called with unexpected argument ‘ruby’, at /tmp/nix-build-nixpkgs-tarball-16.09pre1234.abcdef.drv-0/nixpkgs/lib/customisation.nix:56:12
````