Revert change to remove mathzoom from khan-exercises

Summary:
The change didn't have the effect I expected; in testing, this change seemed to stop MathJax from attempting to load MathZoom.js (which used to cause a 'File failed to load' error in the iOS app), but after landing & testing again, the error returned.

Reverting instead of updating webapp to point to the version of khan-exercise with the rebuilt mathjax.

Test Plan:
Check out problems like 'finding absolute value' and 'subtracting within 1000' in webapp; they still load!

I can't check the behavior in the iOS app right now because exercises crash on load in Xcode 7, and I suspect Xcode 6 is failing to build while Xcode 7 is open looking at the project (while Xcode 7 downloads iOS 8 simulators). I'd like to land this before checking on iOS, though, so we don't risk someone bumping the k-e submodule without updating MathJax references in socrates & the webapp html template.

Reviewers: kevinb, emily, jared, james

Reviewed By: james

Subscribers: james, #ios

Differential Revision: https://phabricator.khanacademy.org/D22127

build/kathjax.py

@@ -9,6 +9,7 @@
 pack = [
     'extensions/MathEvents.js',
     'extensions/MathMenu.js',
+    'extensions/MathZoom.js',
     'extensions/TeX/AMSmath.js',
     'extensions/TeX/AMSsymbols.js',
     'extensions/TeX/noErrors.js',

khan-exercise.js

@@ -2143,7 +2143,7 @@ function loadMathJax() {
     if (window.MathJax) {
         waitForMathJaxReady();
     } else {
-        loadScript(urlBase + "third_party/MathJax/2.1/MathJax.js?config=KAthJax-f44d133888282e24c89e102b0a3fa99c", waitForMathJaxReady);
+        loadScript(urlBase + "third_party/MathJax/2.1/MathJax.js?config=KAthJax-3d6f4e415c7ff2242f5279bfbcbb5c9f", waitForMathJaxReady);
     }
 
     function waitForMathJaxReady() {