I stubbed this method to make these spec examples to cover 2 types of test cases. 
* One is a case where `bundle` is not suffixed or prefixed, 
    * "uses bundle exec to find cucumber and libraries" example
* and another is a case where `bundle` is suffixed and/or prefixed.  
    * "obeys program suffix for bundler" example

If it directly calls the `Gem.bin_path`, it will return the path of `bundler` on the user's  gem environment where this spec was run.  And without a stub, it will not be able to control which flavor of the `bundle` path is returned.