Protect against failure to load MathJax

- e.g. if on a train and don't have access to S3 or the rest of the web

common/lib/xmodule/xmodule/js/src/capa/display.coffee

@@ -11,6 +11,7 @@ class @Problem
     $(selector, @el)
 
   bind: =>
+    if MathJax?
       @el.find('.problem > div').each (index, element) =>
         MathJax.Hub.Queue ["Typeset", MathJax.Hub, element]
 
@@ -31,6 +32,7 @@ class @Problem
 
     # Dynamath
     @$('input.math').keyup(@refreshMath)
+    if MathJax?
       @$('input.math').each (index, element) =>
         MathJax.Hub.Queue [@refreshMath, null, element]
 
@@ -230,6 +232,7 @@ class @Problem
               showMethod = @inputtypeShowAnswerMethods[cls]
               showMethod(inputtype, display, answers) if showMethod?
 
+        if MathJax?
           @el.find('.problem > div').each (index, element) =>
             MathJax.Hub.Queue ["Typeset", MathJax.Hub, element]
 
@@ -273,7 +276,7 @@ class @Problem
     preprocessor_tag = "inputtype_" + elid
     mathjax_preprocessor = @inputtypeDisplays[preprocessor_tag]
 
-    if jax = MathJax.Hub.getAllJax(target)[0]
+    if MathJax? and jax = MathJax.Hub.getAllJax(target)[0]
       eqn = $(element).val()
       if mathjax_preprocessor
         eqn = mathjax_preprocessor(eqn)
@@ -286,6 +289,7 @@ class @Problem
       $("##{element.id}_dynamath").val(jax.root.toMathML '')
     catch exception
       throw exception unless exception.restart
+      if MathJax?
         MathJax.Callback.After [@refreshMath, jax], exception.restart
 
   refreshAnswers: =>