Use completing-read instead of ido-completing-read

I'd like to thank @dougm for his multi-machine work, it's very handy.  As a tweak, I'm proposing using plain `completing-read` instead of hard-coding `ido-completing-read`.

Ido users will still get the behaviour they're used to via hooking into `completing-read-function`, and similarly helm users can still use helm.  Explicitly using `ido-completing-read` forces `ido` on everyone and removes this flexibility.