Change a local copy of completion-at-point-functions, not the global one

As with dgutov/robe#4, ac-js2 currently clobbers the global `completion-at-point-functions` list, which messes up completion in the minibuffer.

This fix is analogous to that applied by @dgutov in https://github.com/dgutov/robe/commit/28965cc552b01b21f9028118d1fe5143d3c04a5e

Cheers,

-Steve