FYI, under Ruby 1.8.7, passing `nil` to `Readline.completion_proc=` causes an `ArgumentError`. This happens when `use_path_completion?` and `completion_options.any?` both return false. Later Ruby versions handle `nil`.

You can see an example at https://travis-ci.org/bundler/bundler/builds/15727954.

I can work on a fix for this.