GutenYe/rubygems-bundler — comment on commit 19fea065283f0589610ddc5847bb5a92a9cf076e
File: rubygems-bundler.gemspec line 23
By mpapis:

then it's an issue with the above installer, you could play with it more and maybe log outputs, or internal variables to check what happens on your system that forbids it from installing the wrapper

and as wrapper is already stable you could install it manually (just copy it) as I do not expect it to change often.