make Primer the default interface

It was supposed to be like this already. (see #936)

I ran across this when setting up a clean install under a VM. I also saw [a recent article][1] claim that Bezel was the default and was going to correct them, but…

Primer is correctly set as the fall-back if nothing is defined, but since there's an entry in the defaults, it's always defined.

[1]: http://mac.appstorm.net/reviews/productivity-review/quicksilver-the-best-free-way-to-do-everything-with-just-your-keyboard/