Remove MathZoom from kathjax.py, and rebuild MathJax

Summary:
MathZoom was still being linked from MathJax, and mobile clients were showing a "File failed to load" error every time a user first loaded an exercise. This removes (hopefully) the last reference to MathZoom.

Still TODO: Once this is landed, update socrates.js with the link to the appropriate KAthJax config file.

Addresses https://app.asana.com/0/15238128244105/52923964249346.

Test Plan:
Load a new khan-exercises exercise in iOS ('finding absolute value' is favorite); you can interact with the exercise and the 'file failed to load' error no longer shows.

Make the same changes in webapp (remove the MathZoom reference & rebuild), and khan-exercises still load & show progress.

Reviewers: emily, kevinb

Reviewed By: kevinb

Subscribers: andy, #ios

Differential Revision: https://phabricator.khanacademy.org/D22079

build/kathjax.py

@@ -9,7 +9,6 @@
 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-3d6f4e415c7ff2242f5279bfbcbb5c9f", waitForMathJaxReady);
+        loadScript(urlBase + "third_party/MathJax/2.1/MathJax.js?config=KAthJax-f44d133888282e24c89e102b0a3fa99c", waitForMathJaxReady);
     }
 
     function waitForMathJaxReady() {