bundler: fix usage on nested call

The combination of bundler 1.12.5 and rubygems 1.6.2 doesn't play well
at all when trying to run gems such as foreman where bundler is used to
run bundler.

Just upgrading to the latest bundler rc doesn't fix it and changing rubygems causes a massive rebuild.

Issues:

- https://github.com/bundler/bundler/issues/4402
- https://github.com/bundler/bundler/issues/4576
- https://github.com/bundler/bundler/issues/4602
- https://github.com/docker-library/ruby/issues/73

This PR patches bundler to work around the issue as highlighted here and
unbreaks everything for me:

https://github.com/bundler/bundler/issues/4602#issuecomment-233619696

pkgs/development/ruby-modules/bundler/default.nix

@@ -7,4 +7,8 @@ buildRubyGem rec {
   version = "1.12.5";
   sha256 = "1q84xiwm9j771lpmiply0ls9l2bpvl5axn3jblxjvrldh8di2pkc";
   dontPatchShebangs = true;
+
+  postFixup = ''
+    sed -i -e "s/activate_bin_path/bin_path/g" $out/bin/bundle
+  '';
 }