Updated khan-exercise to fix dependencies in 'congruency' utility

Found a discrepancy in utils/congruency.js.  The require() statements:

require("./angles.js");
require("./interactive.js");
require("./graphie-helpers.js");

did not match the dependencies in moduleDependencies from khan-exercises.js:

"congruency": ["angles", "interactive"],

Thus failing to populate Khan.modules with:

expressions
graphie-helpers
math-format

This bug shows in exercises/congruent_angles.html, when we compare Khan.modules to what moduleDependencies / resetModules would produce.

Found when doing unit tests in preparation for pushing:
https://github.com/pjmattingly/khan-exercises/tree/add_page_load

(end-to-end tests on exercises)