change how executable path is setup

[ckhall]

fixes an issue where self.executable_path was returning a list, so the returned command was

[['/path/to/ruby'], '-S', ...]

instead of

['path/to/ruby', '-S', ...]

[ckhall]

I'm not much of a python dev, so feel free to make it better if necessary. This fixes the issue I was having.

[ckhall]

any chance this can get reviewed/merged soon?

[martinbohman]

I need this too!