Ensure a valid github profile is chosen.

I was seeing the following error when I had more than one github profile:

    Invalid slot type: gh-gist-api, :base, string, nil

I tracked it down to this completing-read not requiring valid input.
This change makes it so:
* If the user inputs nothing, it returns the first profile in the list.
* The choice must be in the list of profiles.

Meanwhile, for people using ido-ubiquitous, this is also an acceptable
workaround:

(setq ido-ubiquitous-enable-old-style-default nil)