Change _.super to _super.

"super" is a reserved word as of ES6. This makes jshint very unhappy. This
change fixes up all our call sites to call it "_super" instead. Once this is in
I'll push a trivial change to rb-extension-pack to update its use of _.super.

Testing done: Ran js-tests and jshint.

Reviewed at https://reviews.reviewboard.org/r/5600/